You have asked a potentially "hot" question my friend. My experience is that Disney does it a bit differently than other resorts and typically does not upgrade guests that ask for an upgrade. The check-in cast member will be polite, but will not upgrade you, even if you offer to pay the difference. Most Disney "upgrades" are based on WDW's need for your room, or in most cases, your room category is unavailable at check-in, so lucky you, will get an upgrade. Once in awhile folks will post on this board that they were upgraded but those are really the exception not the norm, and who knows, maybe their room category was not available, so it appears they are being upgraded.
Best of luck to you and enjoy your Disney vacation.