In Warlods of Draenor, Moroes appear in our garrison Inn as a quest giver. He does complain of the mess we did last time we went into Karazhan and how long it took him to clean up the mess and he is still cleaning... So he asks us to get him some acid he needs to make a super cleaner.
So, it seems to me that the events that happened in Kara are, in fact, cannon. He aknowledges we were in there, we did stumble tru the tower and did a mess. As of him being alive.. Well... He is an undead and his spirit is anchored to Karazhan, just like many others. So if you want an explanation, i think is pretty simple. Even tho we defeated him, that dosent mean he wasnt able to stand up again after, lets say, some recovery time :P
Now id like to say, to all those people bitching about "re-used" content, that many of us been asking for this very thing... that's it... a reazon to go back to Karazhan. The revamp of the Deadmines was one of the best, the dungeon didn't change, just the story did move foward. Scholomance revamp was a failure cause they did cut so much of it that it did not feel the same at all. Perhaps Scholo was not the best choice to revamp. Scarlet Monstery was neither a success, but is not as bad as Scholo.
Also don't forget we are not losing the original Karazhan, it is still there. This new dungeon will contain new bosses im sure, with obvious come backs as Moroes, and i'm willing to bet, there wont be a chess event (lulz) and Hopefuly Malchezaar too will be back, but not as final boss. I'd like to see someone bigger that is behind this, a bigger demon that is right now giving orders to Malchezaar, that would be amazing! Anyway lets wait and see. After all, this is NOT a revamp, is a new dungeon, inside the tower we do love. And the old thing, will still be there!