Room upgrades are done at the convenience of the resort and you have no control over them. It can be maintenance related; overbooking; someone who couldn't leave on their check-out day to a weather related issue, etc.
Since Disney has added more room categories, based on prior requests, especially in the moderate resorts, upgrades are getting few and far between.
It is best not to get your expectations and hope you are getting a free upgrade, because chances are it is not going to happen. It is not a great way to start a vacation with high expectations and you don't get something that is completely out of your control.
Best to book what you want and be happy you are at Disney.