I have been doing some web surfing and fell upon CruiseCritic.com on the Carnival Boards I have learned a couple of things:
1. After final payment you can get upgrades for cabins or ship board credit, That is only of the cruise price drops..
Many people have booked 6A guarantees and have ended up in Catagory 11, or upto $400.00 on their sign and sail card.
2. If you are working with a Travel Agent (TA) the TA must request it for you. If they say that Carnival doesnt allow that its not true, sometimes they lose commission if they must drop your price, so insist on that TA to call call and call.
Well Hope that Helps you guys out. It sure has helped me I have saved 300. 00 off my cruise now for the Carnival Spirit in May because of this.