We'll see a fork soon, some nodes run old NRS without AM support...
I suggest have the feature ready two or more versions before the upgrade announcement in the future. That's, we can enable AM in version 0.5.3, 0.5.4 and we announce it in 0.5.5. So I think most people have already updated to 0.5.3, so the fork probability will decrease a lot.
potentially we could write in a feature for the client to periodically check an alias directed to a webpage. This would enable a method to broadcast messages to people to move off the old client or they will go off on a fork. Heh. It could even write some data that causes to crash the client at block 44000/whatever.