As long as the resort isn't full, yes. However, that means that you will possibly give up the room that the assignor gave you, if it hasn't been cleaned yet. If you had several requests on your ressy, you might not want to give up the room he set up for you. If you fly in at 10:30, you'll get to Disney about 11:15 - 11:30. Your room may very well be ready at that time anyway.
If you do get your room early, don't forget to have a couple bucks ready for the bellhop, who's brining your bags in (that may take a few minutes after you arrive).