I have an existing Rails 3.1rc4 app deployed to heroku on the Bamboo stack. I would like to upgrate to 3.1rc5 and use the new Cedar stack. Since I can't use heroku stack:migrate, I have to create a new stack and push to that. The problem is that my current app is located at appname.heroku.com. If I simply delete this app, and recreate it using Cedar, then it would become appname.herokuapp.com. How would I move to Cedar while still keeping appname.heroku.com?
Easy, just rename your old app to something like appnameold then create a new one with heroku create appname --stack cedar. Heroku takes care of the redirecting automatically and your previous links shouldn't break. You can Google for herokuapp.com site names and test them if you want to verify that.
If your question is how to have both subdomains separately or you simply like the look of appname.heroku.com over appname.herokuapp.com, however, you're out of luck.