As you all know, Apple changed how Xcode works such that you can\'t ever create an app with a previously used app ID, even if you\'re just writing test apps to use locally.
Just use the bundle ID of any old app that you have already installed like this and change the bundle ID after 7 days. I think apple puts a limit for free developer accounts.
EDIT: Its important about what @haquangvu mentioned in his answer that, your old app get replaced. So take care of it and thanks @haquangvu for your answer.