Why cant I faction change to Ally side?

Hey! I got max level warrior on horde side (ud) and I have few times thinked about transferring to ally side. When I go to services and choose faction change there why I cant see my warrior there in transferrable characthers? Is there some reqs for transferring from horde to ally cuz there is way more hordes than allies you should give even free faction change to mix up that.

You need to make a char on ally side with the class and look you want. (f.x make a gnome warr with a certain look)

Then you can select your old char and the new char you want it to look like.