It's the luck of the draw, alignment of the planets, kind of thing. Depending on the time of year you go sometimes just by asking nicely you can get an upgrade.
You don't get upgraded near as much as it sounds on these boards. You can ask at the desk how much it would cost to upgrade and then see if they offer to do it for free.
Don't count on it and if it happens just consider it part of your magical experience at WDW.