Actually, I just learned this last year.
I don't know how Carnival or any other line does it, but I don't believe they "notify" you directly, if you purchase the cruise through them. I think you have to check this out for yourself.
Actually I just made reservations for next March through my TA. In the email I sent her, I mentioned (I wanted to make sure it was in "writing") that if the price should go down before we cruise, that she would make sure we get the "upgrade" or "discount."
Actually, what I did, was checked about 3, 2 1 month before you sail. If the prices go down and you have booked through Carnival (be sure and get a "name" of the person you booked through), you can email them, but I think when you "call" them, then you don't get all the hastle or the feedback of delay, that you would have through the email.
This is what I did, I called Carnival or whatever line you have booked through, "acting" like I was a "prospective customer", and in doing so, they quoted me the price of what I was looking for. You must take into consideration that there will be port charges/taxes/transfer fees "added" on to the price of the room.
If you book through a TA, they will not be checking on prices for you, so you must contact them, and tell them that the price has gone down, after checking with the cruise line.
I hope this will help. If you have any other questions, please feel free to ask. Actually we (by me checking ever so often (the prices may NOT go down, so you just have to keep checking as it gets closer to traveling time)save about $300, which we were able to spend
Just as another thought....you would be better off, getting insurance to cover you, for "just in case", something might happen. I'm not trying to scare
you, but twice things have happened to us, so it was worth having insurance to cover us.