I versioned the path which is reflective of the build. Each build at least when the domain objects get changed will create a different serialization signature, which when versioning the remote servlet path to reflect the same version of the client version. This way multiple versions could exist with in the same system. Although this is easier to deal with in tomcat context, I haven't explored multiple version compilations with GAE, but I would imagine there are 10 slots per app, which could be used for versioning. Rest is probably more tolerant b/c of the key value mappings, but it has a versioning system for differences in future releases.

https://github.com/ArcBees/CarSample/blob/master/src/main/java/com/arcbees/carsample/client/util/PhoneGapDispatchAsync.java#L14