We have been real lucky, so far, when booking like that via RCCL. My only fear is getting a room too far forward or aft. If that is not a concern, go for it. The worse we got was a 4 cat jump up and the best was a 7 cat jump up but please be aware that you may not be upgraded at all.
No idea if it helps but being C and A Diamond members may have an influence on said upgrades.