I thought I would repost my reply on the "active duplicate" thread.
The rule is never. But that isn't actually true.
My Aunt made ressies with her points as a gift for her sister (my other Aunt) and family for 2 nights in 2 studios at VWL in August 2001. They were on the way to the
Disney Cruise and stopped by WDW for 2 nights.
When my other Aunt arrived at VWL, they told her they were overbooked and upgraded her to 2 one bedrooms instead!!
Don't know what actually happened since it really shouldn't have. Maybe they overbooked on the cash ressies or maybe they just had open inventory available. It may have helped that they checked in late and where there for a very short trip. Who really knows? All I know is that they got extremely lucky!!
DA said it was beautiful there. Her only complaint was that she shouldn't have paid for park tickets for the day - the kids want to leave the room or pool and had to be dragged to the park!
So, bottom line is even though it's not impossible, DON"T COUNT ON AN UPGRADE!!!