I've been hearing rumors that if you race change an existing lvl80 character to goblin (or worgen for that matter) when Cata hits you'll be reverted to lvl1 to make it fair for everyone. Now I'm not actually believing it but I don't remember seeing any news regarding this.
I am not sure if it is possible to make that change on the 7th. I can't see them taking your money and then dropping your character to level 1. Although considering the current leveling speed why spend the money on a race change?
Only thing to make it "fair" is that the realm first achievements require you to do the starting zones of Worgen/Goblins. And that´s only possible if you create one at level 1 since they´re instanced like the DK starting zone.