Most of the time cruise lines will only offer an upgrade from inside to inside or ocean view to ocean view etc. Carnival does not, as far as I know, offer upgrades to anyone purchasing a category 1A ticket. You usually have to book as a minimum, cat. 4A accommodations.
With respect to rebates, my agency keeps a constant watch to see if the prices on any of the cruises that we have booked go down. (special promotions etc) In that situation, we will call the cruise line to try and revise the booking so that our customers will get a lower price. The statement that cruise lines do not give rebates is true unfortunately.