Config files like app.php bootstrap.php

When you upgrade along from let’s say 3.2 all the way to 3.9, but there are important changes made inside. Will the original config files still be correct, or will the core detect that you need to make changes to your config files when updating through composer?

There will be deprecation notices.

However best to see/copy files from cakephp/app to carry changes like this or this

Also see 3.x migration guides