CoreData or "never change an existing data model"

Ok, so sometimes I'm a little bit naive (esp. regarding SW development):
Added a new entity to my CoreData data model, build&run - hups.
App crashes, the console contains

"The model used to open the store is incompatible with the one used to create the store"

Googling around for the error message quickly brought the sholution - delete app + data from the iPhone simulator, and re-build it. Everything is fine again :-)