I think it's worth it to call. They will usually adjust the price. It happened to me two times. Once they gave me an on board credit for the difference and once they actually charged my credit card the lower rate. I usually book early and they have been very flexible.
You also have to be sure it is the same type of booking. If it's a different promotion you may not qualify. Won't hurt to check into it.
Carnival Sensation 02/2003
Carnival Miracle 09/2006
Carnival Inspiration 08/2007