Was your data in the Appengine Datastore? I believe elicox was asking about using Google Cloud SQL, which I believe makes data export relatively easy (Cloud SQL is basically managed MySQL). New Django + Cloud SQL also runs basically "pure" Django, rather than the hacked-up djangoappengine of the past, so next to zero code should have to be changed in a move off AppEngine.

I recently made the switch from Datastore/djangoappengine to Cloud SQL, and it has been a far better experience.