Hold on, so an item you bought with RW currency is NOT usable in the next expansion? Where oh where have I seen this before?
Pause...pause....pause
Oh yeah, 'Warlords of Draenor' (Activision / WoW). Want to know how that went over?
Two words: "Not" and "Well". In fact, the notion of NOT being able to use your flying mounts on / in Draenor went so poorly, you COULDN'T unsubscribe. You had to talk to a CS rep (back when they had them) first.
What came out of that boondoggle? Pathfinder. A thoroughly reviled EXTRA system.