Migrations are not supposed to be changed once they've been merged. The whole idea of migrations is to provide a history of what changes were made and when. Even if the migration is not in a release version, once it is merged the idea is to not change it unless absolutely necessary.

This allows boards that are already installed to not have to reinstall, but can simply update. If you edit a current migration, that migration will be listed as already run, so the change will not be reflected in the installed board. But if you make a new migration, all you have to do is upload that new migration to the board and run database_update.php and the changes are made.