If you google Carnival cruises one online agent mentions you are allowed a free change but not sure if that’s correct if you book direct, I know a long time ago you could do that with p&o Australia and haven’t heard of it for a long while, I can’t mention the online agent but might be worthwhile to ring Carnival and ask them….. I moved a cruise recently and was not charged a cancellation fee but the person said they normally don’t do it