We have been going to WDW since 1971 almost every year. Many years for the past 25 we have gone three to four times each year. During all those hotel stays with WDW, we have only been upgraded twice. The first time (probably about 10+ years ago?) we were put in the last room available at ASMu and it was badly damaged. It was 2:00 am when we discovered the problem! We were eventually moved to CSR, to a standard view room there. We didn't get to our room until after 5:00!!! That was painful! We lost the better part of our first day to that 'upgrade'. Up all night from 1:00 am to 5:00am checking in after travelling all day to get there was so tiring! WE slept that morning away and was tired for the rest of the day.
The other time we were staying in BWI CL (2 years ago) and my Mom experienced an allergic reaction to something outside her room (Plants/trees we weren't sure what it was but she couldn't stay there). Again, it was her the end of the evening when we arrived (about 10:30 pm). Again, no where to move us. So they found a room at the AKL. It was a 2 bedroom suite! That was an upgrade that I would have liked to pass on, but my Mom needed to move. When we arrived we were at the far end nearest to Kidani, but in Jambo house. It felt like forever to walk that long hallway, cross the lobby and take the elevators to the AKL lounge. I had a cold at the time and wasn't feeling well either, so we would stop to rest on benches along the way. It was a lovely suite, but it was bigger than we needed. I would have preferred a regular room near the CL, but that wasn't an option.
neither of my 'upgrades' would have been my choice. I would have preferred to have the room that I booked.
Upgrades DO occur, but they are rare. It would most likely be when the room type that you have booked is unavailable or if there is a problem with your room. If your room type is unavailable, they can, and sometimes do, DOWNGRADE you to something less than you booked. That can be really disappointing.
Our strategy is always book what we want or will make us happy. My dream is to get a nice room in the category that i booked with everything like balconies and daybeds. You can't always get that.