2 experiences to share.

#1. YVR - HKG, I was upgraded to J. There's some mechanical issue with the aircraft after pushed back so we were on the tarmac for 30 minutes. Then pilot announced that they would return to the gate and offload all pax. There was 10 hours delay. When flight finally took off, I noticed one of the J pax sitting next to me was not there. (Perhaps he decided not to fly that day or made alternate arrangement). Instead another pax sat there. So not sure if they run the U/G list again. The app still showed the original U/G list.

#2 MAD-YUL. We were upgraded. One to J, one to PY. We were the last to board in case there was a no-show for J and the other one could get lucky. Shortly after we boarded, the captain said boarding completed. I noticed there 2 J seats, one behind me and one to the left of me. I asked FA and was told that there were 2 deadheading pilots flying today. My partner flying in PY noticed that someone from PY and also someone from Y went up to J, presumably sat in those 2 empty J seats. I noticed there were about 12 staff on standby list for Y and PY and J U/G list. My suspicion is that there since boarding was completed and they were 2 empty seats, the staff on standby end up with those 2 empty J seats. I didn't help that you could only be U/G once otherwise my partner could have taken one of those J seats.