Yes, if you watch the prices on Carnival's website and the price on your cabin goes down just call your PVP and ask for the new price. They should keep up with prices and do this automatically, but I wouldn't count on it !
I know it's absolutely none of my business, but why did you pay in full already? That $$ could have been in an account making $$ for you instead of Carnival!
"A truly happy person is one who can enjoy the scenery on a detour."