Don't count on upgrades.
Disney has added a lot more categories, due to extensive "requests" in order to get you to pay for what you really want.
They are few and far between and totally out of your control. They occur when a resort is overbooked, people have to stay an extra day due to weather at their destination, maintenance, rooms out of inventory for refurbishment.
Best to book what you want and be happy that you get it. I wouldn't expect to get one - it is not a good way to start up your trip by getting your expectations up too high.