If they give a free recustomization, you will be able to change your character's gender as well, basically giving you access to another model for free.
They can't code a change for appearance that does not affect gender yet.
So i don't know what they will decide to do about that.
People will surely pay a lot for recustomizations or even race changes. There will be a lot of money from that.
There's no way they are going make us all pay for the new customizations. I believe that when WoD pre patch hits, the characters will then be given the option to change their appearance.