You can upgrade at the desk, but it will cost you. Numerous people have stated that whatever rate they quote for the upgrade it is negotiable and that you should take multiple shots at getting them to reduce the price. When I was there in June they quoted me 200 per night to go from an ocean view to a Bash tower room. I said "I know you can do better then that" and they lowered their quote to 120. It wasn't that important to me (and I hate negotiating), so I simply said no and they didn't make another offer. Others who are better at this sort of thing have reported getting them to go lower.
We have booked through RCI twice, once for Temptation and once for Desire RM. We were given a room upgrade and just paid the all-inclusive fee.
We're also going to give it a shot at the front desk when we arrive and see what they say. Kim is in charge of all negotiations as she has 100% success when she is negotiating with me sooo what could go wrong....right?