I am Platinum on Carnival and the only time I received an upgrade was...two weeks before I cruised, my TA called and told me that Carnival had buyers for two balcony connecting rooms and asked if we would upgrade to a Cat. 11 which is a suite so they could have our staterooms.
I was also told by a Carnival Rep that upgrades are given first to passengers who have paid full price for a cruise.
Lots of people on these Boards claim that they have received upgrades numerous. If you're moving from an interior on the Riviera Deck to an interior on the Upper Deck, I don't call that an upgrade, they're just moving you around and the location of that stateroom on the Upper Deck can be close to a noisy area of the ship or even worse, near the galley. If someone books a balcony room and wants an interior room across the hall for their children, they will ask you to move from that interior room to accommodate the guests who are purchasing two staterooms.
It's all a crap shoot when it comes to upgrades. We always book a stateroom in the location we want because the upgrade fairy rarely shines on us.