What's the correct way to rename a heroku app?
Did you try heroku apps:rename NEWNAME?
Why use all caps?
heroku apps:rename my_new_app_name
Here is a great place to find out more:
How to rename your Heroku application
In case anyone is curious this method does not require any further configuration, your modifications should be immediate. At the time of this writing I was able to change the name of my app in seconds, and then do so again if I wished. Hope this helps.