Question about race change in Cataclysm

According to http://us.battle.net/wow/en/blog/1116854 we will be able to race change into the two new races, worgen and goblins. I plan on doing this, but my question is would I be able to go ahead and pay for a race change now, and be able to use it right on Dec 7th after my account is upgraded to change my character to one of the new races? The purpose of this would to be to avoid a huge que (and possible lock out) when the expansion actually hits. However, I don't want to do this if there is going to be some weird rule/bug if you pay for the race change now, before Cataclysm is out. Thanks for the clarification.