Well, it's been 19 yrs and 9 yrs ago that my wife and I enjoyed our previous cruises, both with Carnival, and both booked 12+ months in advance. On each of those cruises, we were given an upgrade that we were advised about by our travel agent about a week prior to selling. In each case, I had booked the lowest deck with an outside (window) cabin, trying to save money while still being able to see daylight. There weren't many options back then to have balcony rooms unless you were booking Royal Suites, but we WERE upgraded to a slightly larger stateroom a few decks higher. It was explained to me that since we booked so early, and there was a higher demand for the "more affordable" staterooms, the cruiseline would rather give me the free upgrade and be more likely to re-sell my stateroom reservation to someone that would opt NOT to cruise at all if they couldn't get the more affordable one.
We set sail on Aug 12 with RCI Navigator of the Seas, two adjoining staterooms, so I don't expect any upgrade at this point... plus, the only upgrade available now would be one of the suites, and I prefer to have two adjoining rooms so we have two bathrooms!