Not unusual in the cruise industry. Sounds like you may have been upgraded. They do this to be able to fill the ship. As the cruise date approaches, they will move booked passengers from less expensive cabins to the more expensive cabins so that they can offer "specials" on the cheaper cabins to fill the ship. BUT, just because they moved you up a deck does not necessarily mean you got upgraded because the same cabin category can exist on multiple decks. The only way to know is look on the deck plans on the website. If you know your old cabin #, look and see what category it was, then compare that with the new cabin number.
Normally they do this when you book a "gaurantee" cabin (where they gaurantee a minimum category). But in this case they did it with a stated cabin preference.
If you did get upgraded, great! Have a wonderful cruise.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007