A different UI is not necessarily a worse UI. You are right in a different UI forces users to discard what they've learned. That means that the new UI has to a

A different UI is not necessarily a worse UI. You are right in a different UI forces users to discard what they've learned. That means that the new UI has to a whole lot better than the old UI, to make it worth the pain of the learning process. Case in point, the mouse was a much different UI than what we had before, but it has been proven to be far superior, and worth the change. changing the UI just for the sake of change, that is bad for the user. Change it because it will be better (and not just marginally better)