Lazy Blizzard with Blood Elf NPCs

I'm sure everyone has noticed that since the patch Blizzard have changed the Blood Elf NPC models to that of the new playable race that is coming in The Burning Crusade. Draenei NPCs have also been introduced in various areas in the game. Excellent! We get to preview the new races before the expansion is released.

However, why oh why have Blizzard not changed ALL the Blood Elf models? Sure they did the whole of Azshara, you can get a good laugh as a priest mind controlling the mobs to see their casting animations and their funny twirly jump... But anyone who's teleported to Molten Core recently will have noticed the Blood Elf NPC there is still the same old one from before!

Also, go to Dire Maul and the Blood Elf boss there, Lethtendris, has the new model. But then go to Scholomance and the Blood Elf mobs and the mini boss Intructor Malicia has the same old model.

I thought perhaps this was a continent specific thing, perhaps all the models on Kalimdor have been changed, but not in the Eastern Kingdoms... but that makes no sense when theres several new Blood Elf NPCs in Undercity and the Blood Elf mage trainer in Stormwind has the new model too.

Any blue poster want to tell me why this is, and if Blizzard ever intend to fix such a lazy error?