We were upgraded from G to C.
Yes, we were able to book them, no problem.
At first we thought we might upgrade due to being at the front of the ship, but then found we liked it there compared to Seabourn, so we canceled our upgrade request (second day of the cruise).
We have had great experiences with guaranteed cabins, save one time in over 25 cruises. If you are easygoing they are a bargain.