You can switch from goblin to gnomish or gnomish to goblin.
I talked to a GM about it. He couldn't tell me anythng other than there was a way to switch, and that it had to do with talking to a guy in tanaris.
What you have to do is drop engineering, relevel it. When you can choose between goblin or gnomish you need to go to tanaris and talk to the guy, read the book and all that.  I won't do this because i droped mining for enchanting once i hit 375 engineering to be able to chant my rings.