WoW Insider noted the following information from Production Director J. Allen Brack:
... one day even the skin and face of your characters [will] be customizable through some sort of paid change, akin to changing your character's name, perhaps. They haven't worked out any details yet, but it certainly does look promising.This is great news for my sister-in-law, who hates her level 70 mage's face (or did a few months ago, when she stopped playing her). It also opens up the possibility that they'll add new faces.
So, technically, if someone wants to make their character into a completely new version (except for gender, race, and class), they will one day be able to do so. New hair, skin, face, features, and name = quite a makeover.