Coldchain

Ok, so my question is pretty simple, I just wanted to know if I have retail version installed, I need to download a downgraded patch version of the game, or I just need to change the realmlist to play?

it's just that my internet speed is not that fast, and I have the retail, what I'm asking is if with my retail I can play just changing the realmlist, or if I need the game to be on another patch