here is how it went down in the blizzard offices.
Developers: Okay, so we will open up Race Change about 2 months after the release of the expansion. Sound good? Okay. Now on to the insta--
Mike: Wait that's dumb.
Developers: Why is that dumb? We don't want realm first 85 Worgen on day 1 right?
Mike: Who the hell cares? So many people will be buying Cataclysm because of the new races, and if they can't switch on day 1, that's bad for us.
Developers: So you're saying--
Mike: It's a double whammy. They buy the expansion, plus they race change or faction change. That's like more money for us without any effort!