I am a 370 Gnomish engineer. I realized how pointless it was to stay Gnomish so i ready up on how to change specs. Everyone only said that all you have to do is go to tanaris and north of steamweel there is a house. You talk to the dude inside and read a book and you change specs. After trying this nothing happened, so i opened a ticket and talked to a GM.  He told me you have to:

1). Drop engineering completely

2). Pick it back up and RE level it to 225 (at this point your still either gnomish,or goblin. and you will notice you cant redo the quest to pick between them.)

3). Go to tanaris and talk to the guy in the house and complete his quest. Than read the book and your changed.

this is easy to say but think about how long it will take just to level it back up. I havent done any of this becaus i have rare plans and such and dont feel like tring to get them all over again.  Blizzard needs to make it so  you can change specs without having to relevel all over again.  Alc and tailoring you can just pay gold to change specs Why do you have to re level engineering?
For as much as they change stuff in the game you would think they would change this...

Well gl and hopefuly this was useful