If you're starting a new project with Acquia CMS, please use Acquia's Drupal Recommended Project instead.
If you initially built your Acquia CMS project by running composer create-project acquia/acquia-cms-project, your composer.json is still valid and will continue to work until the release of Acquia CMS 2.0, at which point you will no longer receive the latest version of ACMS by running composer update.
You should merge the contents of your composer.json with drupal-recommended-project's composer.json in order to ensure compatability with all Acquia CMS versions after a 2.0 release. Don't forget to include any Composer dependencies added after the initial composer create-project was completed.