I have indeed had price adjustments, both before and after final payment. My TA can, and does handle the adjustments just the same as the Carnival PVC does, so that isn't an issue. As IBC said, they will lower the final payment, if final payment hasn't been made yet, or make the refund as an on board credit if final payment has been made.
I ALWAYS watch the price of cruises that I've got booked, and perhaps 10% of the time, the price goes down before I sail.
Good luck, and have a GREAT cruise.