I've received free upgrades over the years, I'm glad to say.
Got upgraded from a woods view to a courtyard view and from a standard view to a woods view at the Wilderness Lodge before. Don't know why on either case (didn't ask for it and wasn't told I was getting it ~ I was just given those room numbers at check-in).
I got upgraded from the All-Star Music to Port Orleans-FQ once also. I never was told why on that one either. I had a very late arrival (like 1 am) and was asked if I wished to upgrade to a different resort. I certainly didn't pass on the offer.
I've also had upgrades, for a fee, a couple times. I wanted a different room type and asked for availability at check-in.
I went from a courtyard view at the Wilderness Lodge to a concierge room for an additional $140 per night. Not really a bargain since that is the difference in rack rates anyway but it was on an AP discounted room.
Also received a concierge upgrade at the Polynesian once. The story on that one is I was dissatisfied with the location of my lagoon view room (near the monorail) and went back to the front desk. I asked if there was something available in the Tahiti building. The CM offered me a concierge room, as I was informed that the concierge rooms had been relocated to that building due to a refurbishment, for $80 per night. I accepted the offer for my seven night stay. My bonus came when I got the folio at check-out and the cm had only charged me $80 for one night total. Woo hoo!
It just seems to be a matter of being at the right place at the right time for surprise upgrades at no charge. They seem to use that when they are booked on a specific guests' room type or requests.
