I am not quite sure, but this problem might occur if you had the MPQ in your data folder when you applied the wow patch. I read somewhere, that the model change merges with the other MPQs, meaning your only way to revert it would be to delete some MPQs and then run the repair.exe. Alas, I don't know which MPQs you have to delete
But that's just what I heard, never happened to me before