It is possible.
1. Get models and textures of new version.
2. Retro-port models to WotLK
3. Give textures and models the same names as original ones have, so they are overwritten.
- OR go to ItemDisplayInfo.dbc and change all displayIDs of T6 items so they use new textures and models, but that means that you can pretty much also just create new items, and I suppose thats not something you wanted to do anyway.
Done. If you have no idea how to do this, check retro-porting part of board and learn to create a new item with new custom item in world.item_template and Item.dbc displayID in ItemDisplayInfo.dbc. By managing to do so, you will get all required knowledge and experience.