If you make a change after final payment it is basically a cancellation. You will be in the penalty phase. The penalty can range from forfeiting deposit to the entire cruise fare depending on how close to sailing you make the change or cancel.
This is Carnival's cancellation penalties: If you cancel this is what you will forfeit.
6, 7 & 8 day 7 days or less 100% of Total Fare
Cruises Up to 76 days None
75 to 30 days Deposit
29 to 8 days 50% of Total Fare
Basically: Make your changes at least 76 days before you sail.