So i bought a faction change Tuesday night, and went from horde to alliance. Now today after the updates i can do it again O.o.
So i am guessing this is how it works
1. Buy faction after update on Tuesday
2 have another update happen after that before the next tuesday fixing previous things in the patch.
So this can not really be planned, but maybe if you wanna buy something do it on a tuesday so if you want to change back ever you can have it.