I'm not sure how far out you are but you can cancel before the final payment date and still get credit for part of the deposit. This is what was on Carnival's website under the FAQ for early saver. Hope it helps.
• What if I need to make changes to my ship or sailing date?
Ship and sailing date changes are allowed prior to the final payment due date, subject to a $50 service fee per person, per change. Changes made after the final payment due date are subject to standard cancellation penalties.