I'm sorry, but YOU are the one who keeps changing your booking. It is not their job to keep changing things because of changed promotions. That's what they are - promotions. In the T&C they will likely say that they are for new bookings ONLY. Having the cruiseline jump through hoops to change things causes more churn in a system that is already flawed (MSC's IT dept, billing dept, etc).
Off my soapbox for now. Yes, I get is a couple of hundred bucks, but most of us BOOK our cruise and don't look back.