We booked a balcony gty on Carnival Pride, and at first, when we first found out our stateroom, we were placed on a high deck. Then, when the prices went down for out cruise before we sailed, I called them and they gave me a $100 shipboard credit for the difference. After that, they immediately moved us to the lowest 8a cat. We ended up upgrading to an extended balcony on a higher deck after that and just paying the difference.
Moral of the story....Don't ask for a credit when the prices go down unless you want to be dumped to the bottom of the barrell.