diff --git a/app/components/dossiers/edit_footer_component.rb b/app/components/dossiers/edit_footer_component.rb
index b5acc51f24d..ef40ba0669f 100644
--- a/app/components/dossiers/edit_footer_component.rb
+++ b/app/components/dossiers/edit_footer_component.rb
@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 class Dossiers::EditFooterComponent < ApplicationComponent
-  delegate :can_passer_en_construction?, to: :@dossier
+  delegate :can_passer_en_construction?, :can_transition_to_en_construction?, :forked_with_changes?, to: :@dossier
 
   def initialize(dossier:, annotation:)
     @dossier = dossier
@@ -18,7 +18,43 @@ def annotation?
     @annotation.present?
   end
 
-  def disabled_submit_buttons_options
+  def can_submit?
+    can_submit_draft? || forked_with_changes?
+  end
+
+  def can_submit_draft?
+    !annotation? && can_transition_to_en_construction?
+  end
+
+  def can_submit_en_construction?
+    forked_with_changes?
+  end
+
+  def submit_button_label
+    if can_submit_draft?
+      t('.submit')
+    else
+      t('.submit_changes')
+    end
+  end
+
+  def submit_button_path
+    if can_submit_draft?
+      brouillon_dossier_path(@dossier)
+    else
+      modifier_dossier_path(@dossier.editing_fork_origin)
+    end
+  end
+
+  def submit_button_options
+    if can_submit_draft?
+      submit_draft_button_options
+    else
+      submit_en_construction_button_options
+    end
+  end
+
+  def disabled_submit_button_options
     {
       class: 'fr-text--sm fr-mb-0 fr-mr-2w',
       data: { 'fr-opened': "true" },
diff --git a/app/components/dossiers/edit_footer_component/edit_footer_component.html.haml b/app/components/dossiers/edit_footer_component/edit_footer_component.html.haml
index 2f0f59b2b1a..f6a727a3e47 100644
--- a/app/components/dossiers/edit_footer_component/edit_footer_component.html.haml
+++ b/app/components/dossiers/edit_footer_component/edit_footer_component.html.haml
@@ -2,16 +2,10 @@
   .send-dossier-actions-bar
     = render Dossiers::AutosaveFooterComponent.new(dossier: @dossier, annotation: annotation?)
 
-    - if !annotation? && @dossier.can_transition_to_en_construction?
+    - if can_submit?
       - if !can_passer_en_construction?
-        = link_to t('.submit_disabled'), "#", disabled_submit_buttons_options
-      = button_to t('.submit'), brouillon_dossier_url(@dossier), submit_draft_button_options
-
-    - if @dossier.forked_with_changes?
-      - if !can_passer_en_construction?
-        = link_to t('.submit_disabled'), "#", disabled_submit_buttons_options
-      = button_to t('.submit_changes'), modifier_dossier_url(@dossier.editing_fork_origin), submit_en_construction_button_options
-
+        = link_to t('.submit_disabled'), "#", disabled_submit_button_options
+      = button_to submit_button_label, submit_button_path, submit_button_options
 
   - if @dossier.brouillon? && !owner?
     .fr-pb-2w.invite-cannot-submit
@@ -19,7 +13,5 @@
         - c.with_body do
           %p.fr-pb-0= t('.invite_notice').html_safe
 
-
-
   - if !annotation?
     = render partial: "shared/dossiers/submit_is_over", locals: { dossier: @dossier }
diff --git a/app/components/editable_champ/piece_justificative_component.rb b/app/components/editable_champ/piece_justificative_component.rb
index 47b873e8d51..a0d1f944f75 100644
--- a/app/components/editable_champ/piece_justificative_component.rb
+++ b/app/components/editable_champ/piece_justificative_component.rb
@@ -14,7 +14,7 @@ def view_as
   end
 
   def user_can_destroy?
-    !@champ.mandatory? || @champ.dossier.brouillon?
+    true
   end
 
   def max
diff --git a/app/controllers/users/dossiers_controller.rb b/app/controllers/users/dossiers_controller.rb
index fbc1648889b..2e82070d973 100644
--- a/app/controllers/users/dossiers_controller.rb
+++ b/app/controllers/users/dossiers_controller.rb
@@ -9,11 +9,11 @@ class DossiersController < UserController
     layout 'procedure_context', only: [:identite, :update_identite, :siret, :update_siret]
 
     ACTIONS_ALLOWED_TO_ANY_USER = [:index, :new, :transferer_all, :deleted_dossiers]
-    ACTIONS_ALLOWED_TO_OWNER_OR_INVITE = [:show, :destroy, :demande, :messagerie, :brouillon, :submit_brouillon, :submit_en_construction, :modifier, :modifier_legacy, :update, :create_commentaire, :papertrail, :restore, :champ]
+    ACTIONS_ALLOWED_TO_OWNER_OR_INVITE = [:show, :destroy, :demande, :messagerie, :brouillon, :submit_brouillon, :submit_en_construction, :modifier, :update, :create_commentaire, :papertrail, :restore, :champ]
 
     before_action :ensure_ownership!, except: ACTIONS_ALLOWED_TO_ANY_USER + ACTIONS_ALLOWED_TO_OWNER_OR_INVITE
     before_action :ensure_ownership_or_invitation!, only: ACTIONS_ALLOWED_TO_OWNER_OR_INVITE
-    before_action :ensure_dossier_can_be_updated, only: [:update_identite, :update_siret, :brouillon, :submit_brouillon, :submit_en_construction, :modifier, :modifier_legacy, :update, :champ]
+    before_action :ensure_dossier_can_be_updated, only: [:update_identite, :update_siret, :brouillon, :submit_brouillon, :submit_en_construction, :modifier, :update, :champ]
     before_action :ensure_dossier_can_be_filled, only: [:brouillon, :modifier, :submit_brouillon, :submit_en_construction, :update]
     before_action :ensure_dossier_can_be_viewed, only: [:show]
     before_action :forbid_invite_submission!, only: [:submit_brouillon]
@@ -233,17 +233,9 @@ def submit_brouillon
 
       if @dossier.errors.blank? && @dossier.can_passer_en_construction?
         @dossier.passer_en_construction!
-        @dossier.process_declarative!
-        @dossier.process_sva_svr!
-        @dossier.groupe_instructeur.instructeurs.with_instant_email_dossier_notifications.each do |instructeur|
-          DossierMailer.notify_new_dossier_depose_to_instructeur(@dossier, instructeur.email).deliver_later
-        end
         redirect_to merci_dossier_path(@dossier)
       else
-        respond_to do |format|
-          format.html { render :brouillon }
-          format.turbo_stream
-        end
+        render :brouillon
       end
     end
 
@@ -261,17 +253,7 @@ def extend_conservation_and_restore
 
     def modifier
       @dossier = dossier_with_champs
-    end
-
-    # Transition to en_construction forks,
-    # so users editing en_construction dossiers won't completely break their changes.
-    # TODO: remove me after fork en_construction feature deploy (PR #8790)
-    def modifier_legacy
-      respond_to do |format|
-        format.turbo_stream do
-          flash.alert = "Une mise à jour de cette page est nécessaire pour poursuivre, veuillez la recharger (touche F5). Attention: le dernier champ modifié n’a pas été sauvegardé, vous devrez le ressaisir."
-        end
-      end
+      @dossier_for_editing = dossier.owner_editing_fork
     end
 
     def submit_en_construction
@@ -284,29 +266,21 @@ def submit_en_construction
 
       submit_dossier_and_compute_errors
 
-      if @dossier.errors.blank? && @dossier.can_passer_en_construction?
-        editing_fork_origin.merge_fork(@dossier)
+      if dossier.errors.blank? && dossier.can_passer_en_construction?
+        editing_fork_origin.merge_fork(dossier)
         editing_fork_origin.submit_en_construction!
-
         redirect_to dossier_path(editing_fork_origin)
       else
-        respond_to do |format|
-          format.html do
-            render :modifier
-          end
-
-          format.turbo_stream do
-            @to_show, @to_hide, @to_update = champs_to_turbo_update(champs_public_attributes_params, dossier.champs.filter(&:public?))
-            render :update, layout: false
-          end
-        end
+        @dossier_for_editing = dossier
+        @dossier = editing_fork_origin
+        render :modifier
       end
     end
 
     def update
       @dossier = dossier.en_construction? ? dossier.find_editing_fork(dossier.user) : dossier
       @dossier = dossier_with_champs(pj_template: false)
-      @can_passer_en_construction_was, @can_passer_en_construction_is = @dossier.track_can_passer_en_construction do
+      @can_passer_en_construction_was, @can_passer_en_construction_is = dossier.track_can_passer_en_construction do
         update_dossier_and_compute_errors
       end
 
@@ -324,8 +298,8 @@ def merci
 
     def champ
       @dossier = dossier_with_champs(pj_template: false)
-      type_de_champ = @dossier.find_type_de_champ_by_stable_id(params[:stable_id], :public)
-      champ = @dossier.project_champ(type_de_champ, params[:row_id])
+      type_de_champ = dossier.find_type_de_champ_by_stable_id(params[:stable_id], :public)
+      champ = dossier.project_champ(type_de_champ, params[:row_id])
 
       respond_to do |format|
         format.turbo_stream do
@@ -559,36 +533,34 @@ def set_dossier_as_editing_fork
     end
 
     def update_dossier_and_compute_errors
-      @dossier.update_champs_attributes(champs_public_attributes_params, :public, updated_by: current_user.email)
-      updated_champs = @dossier.champs.filter(&:changed_for_autosave?)
-      if updated_champs.present?
-        @dossier.last_champ_updated_at = Time.zone.now
-      end
+      dossier.update_champs_attributes(champs_public_attributes_params, :public, updated_by: current_user.email)
+      updated_champs = dossier.champs.filter(&:changed_for_autosave?)
 
       # We save the dossier without validating fields, and if it is successful and the client
       # requests it, we ask for field validation errors.
-      if @dossier.save
-        if updated_champs.any?(&:used_by_routing_rules?)
-          @update_contact_information = true
-          RoutingEngine.compute(@dossier)
+      if dossier.save
+        if dossier.brouillon? && updated_champs.present?
+          dossier.touch(:last_champ_updated_at)
+          if updated_champs.any?(&:used_by_routing_rules?)
+            @update_contact_information = true
+            RoutingEngine.compute(dossier)
+          end
         end
 
         if params[:validate].present?
-          @dossier.valid?(:champs_public_value)
+          dossier.valid?(:champs_public_value)
         end
       end
-
-      @dossier.errors
     end
 
     def submit_dossier_and_compute_errors
-      @dossier.validate(:champs_public_value)
-      @dossier.check_mandatory_and_visible_champs
+      dossier.validate(:champs_public_value)
+      dossier.check_mandatory_and_visible_champs
 
-      if @dossier.editing_fork_origin&.pending_correction?
-        @dossier.editing_fork_origin.validate(:champs_public_value)
-        @dossier.editing_fork_origin.errors.where(:pending_correction).each do |error|
-          @dossier.errors.import(error)
+      if dossier.editing_fork_origin&.pending_correction?
+        dossier.editing_fork_origin.validate(:champs_public_value)
+        dossier.editing_fork_origin.errors.where(:pending_correction).each do |error|
+          dossier.errors.import(error)
         end
       end
     end
diff --git a/app/models/concerns/dossier_clone_concern.rb b/app/models/concerns/dossier_clone_concern.rb
index 193561fe41a..6f5aab6f280 100644
--- a/app/models/concerns/dossier_clone_concern.rb
+++ b/app/models/concerns/dossier_clone_concern.rb
@@ -26,12 +26,6 @@ def owner_editing_fork
     find_or_create_editing_fork(user).tap { DossierPreloader.load_one(_1) }
   end
 
-  def reset_editing_fork!
-    if editing_fork? && forked_with_changes?
-      destroy_editing_fork!
-    end
-  end
-
   def destroy_editing_fork!
     if editing_fork?
       update!(hidden_by_administration_at: Time.current, hidden_by_reason: :stale_fork)
@@ -43,6 +37,18 @@ def editing_fork?
     editing_fork_origin_id.present?
   end
 
+  def forked_with_changes?
+    if forked_diff.present?
+      forked_diff.values.any?(&:present?) || forked_groupe_instructeur_changed?
+    end
+  end
+
+  def champ_forked_with_changes?(champ)
+    if forked_diff.present?
+      forked_diff.values.any? { |champs| champs.any? { _1.public_id == champ.public_id } }
+    end
+  end
+
   def make_diff(editing_fork)
     origin_champs_index = project_champs_public_all.index_by(&:public_id)
     forked_champs_index = editing_fork.project_champs_public_all.index_by(&:public_id)
@@ -126,18 +132,6 @@ def clone(user: nil, fork: false)
     cloned_dossier.reload
   end
 
-  def forked_with_changes?
-    if forked_diff.present?
-      forked_diff.values.any?(&:present?) || forked_groupe_instructeur_changed?
-    end
-  end
-
-  def champ_forked_with_changes?(champ)
-    if forked_diff.present?
-      forked_diff.values.any? { _1.include?(champ) }
-    end
-  end
-
   private
 
   def forked_diff
diff --git a/app/models/concerns/dossier_state_concern.rb b/app/models/concerns/dossier_state_concern.rb
index 7b60de6adae..a9ea4c7412f 100644
--- a/app/models/concerns/dossier_state_concern.rb
+++ b/app/models/concerns/dossier_state_concern.rb
@@ -3,6 +3,19 @@
 module DossierStateConcern
   extend ActiveSupport::Concern
 
+  def submit_en_construction!
+    self.traitements.submit_en_construction
+    save!
+
+    RoutingEngine.compute(self)
+
+    resolve_pending_correction!
+    process_sva_svr!
+    remove_piece_justificative_file_not_visible!
+
+    editing_forks.each(&:destroy_editing_fork!)
+  end
+
   def after_passer_en_construction
     self.conservation_extension = 0.days
     self.depose_at = self.en_construction_at = self.traitements
@@ -16,12 +29,18 @@ def after_passer_en_construction
     MailTemplatePresenterService.create_commentaire_for_state(self, Dossier.states.fetch(:en_construction))
     procedure.compute_dossiers_count
 
+    process_declarative!
+    process_sva_svr!
+
     index_search_terms_later
   end
 
   def after_commit_passer_en_construction
     NotificationMailer.send_en_construction_notification(self).deliver_later
     NotificationMailer.send_notification_for_tiers(self).deliver_later if self.for_tiers?
+    groupe_instructeur.instructeurs.with_instant_email_dossier_notifications.each do |instructeur|
+      DossierMailer.notify_new_dossier_depose_to_instructeur(self, instructeur.email).deliver_later
+    end
     remove_piece_justificative_file_not_visible!
   end
 
@@ -49,6 +68,8 @@ def after_commit_passer_en_instruction(h)
       NotificationMailer.send_en_instruction_notification(self).deliver_later
       NotificationMailer.send_notification_for_tiers(self).deliver_later if self.for_tiers?
     end
+
+    editing_forks.each(&:destroy_editing_fork!)
   end
 
   def after_passer_automatiquement_en_instruction
diff --git a/app/models/dossier.rb b/app/models/dossier.rb
index 3d4b9ab9b97..f41be6147e7 100644
--- a/app/models/dossier.rb
+++ b/app/models/dossier.rb
@@ -865,17 +865,6 @@ def email_template_for(state)
     procedure.email_template_for(state)
   end
 
-  def submit_en_construction!
-    self.traitements.submit_en_construction
-    save!
-
-    RoutingEngine.compute(self)
-
-    resolve_pending_correction!
-    process_sva_svr!
-    remove_piece_justificative_file_not_visible!
-  end
-
   def process_declarative!
     if procedure.declarative_accepte? && may_accepter_automatiquement?
       accepter_automatiquement!
diff --git a/app/views/administrateurs/procedures/apercu.html.haml b/app/views/administrateurs/procedures/apercu.html.haml
index 5941fdbb4bd..c847b25b843 100644
--- a/app/views/administrateurs/procedures/apercu.html.haml
+++ b/app/views/administrateurs/procedures/apercu.html.haml
@@ -13,6 +13,6 @@
             active: @tab == 'annotations-privees')
 
 - if @tab == 'dossier'
-  = render partial: "shared/dossiers/edit", locals: { dossier: @dossier }
+  = render partial: "shared/dossiers/edit", locals: { dossier: @dossier, dossier_for_editing: @dossier }
 - else
   = render partial: "shared/dossiers/edit_annotations", locals: { dossier: @dossier, seen_at: nil }
diff --git a/app/views/shared/dossiers/_edit.html.haml b/app/views/shared/dossiers/_edit.html.haml
index c797d35f237..da11f1c03c4 100644
--- a/app/views/shared/dossiers/_edit.html.haml
+++ b/app/views/shared/dossiers/_edit.html.haml
@@ -1,5 +1,3 @@
-- dossier_for_editing = dossier.en_construction? ? dossier.owner_editing_fork : dossier
-
 - if dossier.france_connected_with_one_identity?  && current_user.instructeur? && !current_user.owns_or_invite?(dossier)
   - content_for(:notice_info) do
     = render partial: "shared/dossiers/france_connect_informations_notice", locals: { user_information: dossier.user.france_connect_informations.first }
@@ -10,7 +8,7 @@
   = render NestedForms::FormOwnerComponent.new
   = form_for dossier_for_editing, url: brouillon_dossier_url(dossier), method: :patch, html: { id: 'dossier-edit-form', class: 'form', multipart: true, novalidate: 'novalidate' } do |f|
 
-    = render Dossiers::ErrorsFullMessagesComponent.new(dossier: dossier)
+    = render Dossiers::ErrorsFullMessagesComponent.new(dossier: dossier_for_editing)
     %header.mb-6
       .fr-highlight
         %p.fr-text--sm
diff --git a/app/views/users/dossiers/brouillon.html.haml b/app/views/users/dossiers/brouillon.html.haml
index 0acdb89e571..428347dafe5 100644
--- a/app/views/users/dossiers/brouillon.html.haml
+++ b/app/views/users/dossiers/brouillon.html.haml
@@ -9,4 +9,4 @@
     .fr-container
       = render partial: "shared/dossiers/header", locals: { dossier: @dossier }
 
-  = render partial: "shared/dossiers/edit", locals: { dossier: @dossier }
+  = render partial: "shared/dossiers/edit", locals: { dossier: @dossier, dossier_for_editing: @dossier }
diff --git a/app/views/users/dossiers/modifier.html.haml b/app/views/users/dossiers/modifier.html.haml
index 7217909e07a..6a2b986750e 100644
--- a/app/views/users/dossiers/modifier.html.haml
+++ b/app/views/users/dossiers/modifier.html.haml
@@ -7,4 +7,4 @@
   = render partial: 'users/dossiers/show/header', locals: { dossier: @dossier }
 
   .container
-    = render partial: "shared/dossiers/edit", locals: { dossier: @dossier }
+    = render partial: "shared/dossiers/edit", locals: { dossier: @dossier, dossier_for_editing: @dossier_for_editing }
diff --git a/config/routes.rb b/config/routes.rb
index 518f513aaee..7352258ee10 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -380,7 +380,6 @@
         post 'brouillon', to: 'dossiers#submit_brouillon'
         get 'modifier', to: 'dossiers#modifier'
         post 'modifier', to: 'dossiers#submit_en_construction'
-        patch 'modifier', to: 'dossiers#modifier_legacy'
         get 'champs/:stable_id', to: 'dossiers#champ', as: :champ
         get 'merci'
         get 'demande'
diff --git a/spec/controllers/users/dossiers_controller_spec.rb b/spec/controllers/users/dossiers_controller_spec.rb
index 68fe7a83089..2b855d3a6e0 100644
--- a/spec/controllers/users/dossiers_controller_spec.rb
+++ b/spec/controllers/users/dossiers_controller_spec.rb
@@ -836,10 +836,11 @@
     before { sign_in(user) }
 
     let(:procedure) { create(:procedure, :published, types_de_champ_public: [{}, { type: :piece_justificative }]) }
-    let!(:dossier) { create(:dossier, :en_construction, user:, procedure:) }
-    let(:first_champ) { dossier.project_champs_public.first }
+    let(:dossier) { create(:dossier, :en_construction, user:, procedure:) }
+    let!(:editing_fork) { dossier.owner_editing_fork }
+    let(:first_champ) { editing_fork.project_champs_public.first }
+    let(:piece_justificative_champ) { editing_fork.project_champs_public.last }
     let(:anchor_to_first_champ) { controller.helpers.link_to I18n.t('views.users.dossiers.fix_champ'), brouillon_dossier_path(anchor: first_champ.labelledby_id), class: 'error-anchor' }
-    let(:piece_justificative_champ) { dossier.project_champs_public.last }
     let(:value) { 'beautiful value' }
     let(:file) { fixture_file_upload('spec/fixtures/files/piece_justificative_0.pdf', 'application/pdf') }
     let(:now) { Time.zone.parse('01/01/2100') }
@@ -887,14 +888,9 @@
 
       it 'updates the dossier timestamps' do
         subject
-        dossier.reload
-        expect(dossier.updated_at).to eq(now)
-        expect(dossier.last_champ_updated_at).to eq(now)
-      end
-
-      it 'updates the dossier state' do
-        subject
-        expect(dossier.reload.state).to eq(Dossier.states.fetch(:en_construction))
+        editing_fork.reload
+        expect(editing_fork.updated_at).to eq(now)
+        expect(editing_fork.last_champ_updated_at).to eq(now)
       end
 
       it { is_expected.to have_http_status(:ok) }
@@ -923,9 +919,9 @@
 
         it 'updates the dossier timestamps' do
           subject
-          dossier.reload
-          expect(dossier.updated_at).to eq(now)
-          expect(dossier.last_champ_updated_at).to eq(now)
+          editing_fork.reload
+          expect(editing_fork.updated_at).to eq(now)
+          expect(editing_fork.last_champ_updated_at).to eq(now)
         end
       end
     end
@@ -958,14 +954,10 @@
       end
 
       context 'iban error' do
+        let(:types_de_champ_public) { [{ type: :iban }] }
         let(:value) { 'abc' }
 
-        before do
-          first_champ.type_de_champ.update!(type_champ: :iban, mandatory: true, libelle: 'l')
-          dossier.project_champs_public.first.becomes!(Champs::IbanChamp).save!
-
-          subject
-        end
+        before { subject }
 
         it { expect(response).to have_http_status(:success) }
       end
@@ -998,9 +990,7 @@
     end
 
     context 'when the champ is a phone number' do
-      let(:procedure) { create(:procedure, :published, types_de_champ_public: [{ type: :phone }]) }
-      let!(:dossier) { create(:dossier, :en_construction, user:, procedure:) }
-      let(:first_champ) { dossier.project_champs_public.first }
+      let(:types_de_champ_public) { [{ type: :phone }] }
       let(:now) { Time.zone.parse('01/01/2100') }
 
       let(:submit_payload) do
diff --git a/spec/views/shared/dossiers/_edit.html.haml_spec.rb b/spec/views/shared/dossiers/_edit.html.haml_spec.rb
index efccf5c7d25..d2222c027e1 100644
--- a/spec/views/shared/dossiers/_edit.html.haml_spec.rb
+++ b/spec/views/shared/dossiers/_edit.html.haml_spec.rb
@@ -6,10 +6,11 @@
     allow(view).to receive(:administrateur_signed_in?).and_return(false)
   end
 
-  subject { render 'shared/dossiers/edit', dossier: dossier, apercu: false }
+  subject { render 'shared/dossiers/edit', dossier:, dossier_for_editing:, apercu: false }
 
   let(:procedure) { create(:procedure, types_de_champ_public:) }
   let(:dossier) { create(:dossier, :with_populated_champs, procedure:) }
+  let(:dossier_for_editing) { dossier }
 
   context 'when there are some champs' do
     let(:type_de_champ_header_section) { procedure.draft_types_de_champ_public.find(&:header_section?) }
@@ -116,6 +117,7 @@
 
     context 'when dossier is en construction' do
       let(:dossier) { create(:dossier, :en_construction, :with_populated_champs, procedure:) }
+      let(:dossier_for_editing) { dossier.owner_editing_fork }
 
       it 'can delete a piece justificative' do
         expect(subject).to have_selector("[title='Supprimer le fichier #{champ.piece_justificative_file.attachments[0].filename}']")