We only had this happen once, at Paradise Pier in Disneyland (I would guess things work similarly at WDW) and it was weird. Not sure what to attribute it to, really.
When we were checking in, the person knew we were there for my birthday. We had booked a standard room. My husband asked if any upgrades were available. She said yes there were for $XXX (can’t remember how much) extra. We declined since it was sort of a lot of money and totally unnecessary.
She said our room wasn’t ready yet (this was around 11:00am) so we stored our bags and went to the park. We got back around 10-11:00pm and got our bags and went to the room. I think we had gotten a text with the number?
When we got to the room I was feeling disappointed in it since we had two queens and had requested a king. I was being stupid about it, I think I was just over-tired because I didn’t really care that much. But then my husband told me to open the curtains and we had an amazing park (DCA) view room. That shut me up about the queen beds!
So they never said, ‘oh happy birthday here’s an upgrade’ or anything about upgrading us at all but we definitely got an upgraded room. It could have just been what was available, it could have been a nice surprise from the person who checked us in, could have been something else.