Apollo should arrive to all current gen devices. Fragmentation will be caused, which is what Steven Elop (Nokia) and Steve Ballmer (Microsoft) said they would go out of their way to prevent. Mango apps will work on Apollo, so no, they aren't transitioning to a new OS. WP8 = WP7 with more features. Update just like NoDo to Mango. So why do people worry over Mango to Apollo? Change of whole number? Really? Sorry, they aren't going over Windows Mobile's graveyard.
iOS 5 = iOS 4 with more features = iOS 3 with more features.
Same with Android.
They won't be going a different route. That was in the past..
There is no official source to they will be upgradable/not upgradable by Microsoft or anyone else. Nobody whatsoever.