There is a possibility but I would not count on getting upgraded. What type of upgrade are you looking for? You could say, "I would love anything with the best possible view??" or ask them how much it would cost to get upgraded to whatever you are looking for... they may throw some pixie dust your way. It never hurts to ask.
Also, here are some good tips on preparing a room request letter:
http://www.wdwig.com/cro1.htm
Hope it helps!!!