REST webservice with Android client vs RPC

We are still trying to decide between implementing a REST webservice vs going with RPC. Googles eclipse plugin makes it very easy to create an RPC service which is what the main attraction is for the RPC way. However, a REST service would seem to be easier to modify, IMHO, and would also allow for a future iOS client to connect with little or no rework. One concern I would have and maybe this is a problem with RPC also or indeed any Client-Server model is how do you modify your existing service and allow old clients to still work while allowing new clients to use the new functionality. What I mean is can you point me towards any thing we should look out for when modifying a web service so as not to break existing clients or force them to upgrade. Any links you have that would cover this would be appreciated