I have. Rented a studio for one night, got upgraded to a 1br. I would say that the shorter the stay, the better your chances of getting an upgrade are, although the only basis I have for that is my opinion. That said, I would say that your chances of getting any upgrade, especially on rented points, are very slim. Whatever you do, DO NOT ASK for an upgrade when yo get there. There have been reports of
DVC members' points getting deducted by renters asking for upgrades.