Royal Caribbean will not change the rate if you already paid in full, regardless if its final payment or not. I'm on them every other month and from experience it's really near impossible to get them to lower the fare for your room if you paid in full... even if its months away from final payment. If you push the issue enough, they *may* offer OBC of the difference. I just make sure the cruise isn't paid in full until the week of, so much easier.

As for canceling and rebooking, depending on how the reservation was booked, you'll miss out on onboard booking incentives. As for deposit, Carnival and Royal Caribbean now offer multiple fares for the same category of rooms with some of them being non refundable deposits. So like airline ticket, some time it's more expensive to cancel and rebook because you'll loose out with the nonrefundable deposit.