It depends if your app is using a lot of proprietary AppEngine APIs / features. If yes, then you will have to rewrite at least some portions of the app.

Due to the recent AppEngine pricing changes, I moved my app - Review19.com - away from GAE. I had to rewrite it entirely because I used a lot of AppEngine specific APIs.

There are other open source options that "replicate" some of the GAE APIs, but none of them claim to production ready.