I just had a similar problem with Princess which was resolved very easily through my Princess Travel Agent. Our Cruise is in September 2013, and the price dropped for all Cabins so much that a Mini-suite became less expensive than the Premium Balcony Cabin we had originally booked. Princess let us switch Cabins to a Mini-suite and we put $800 into our pockets. Perhaps that's because we had not paid the final payment; it's due later this month. Whatever the reason this time, I was pleased.
I wasn't so pleased when the same thing happened several times in 2012 with Cunard. If only because of that I will never travel Cunard again!