Obviously, I don't know the specifics of your cruise, but if, in fact, you have the Early Saver Fare, Carnival will allow them to reduce the rate, even after final payment has been made. That's one of the big selling points of the Early Saver Fare. I just did this for a client last week and it was not a problem.
As for the onboard credit, again, I can't speak about that because I don't know the specifics on how you got it and who actually gave it to you (cruise line or the agent).
But I would definitely be asking some more questions about it until you get definitive information that satisfies you.