My personal experience of one time only, but I did a same day confirmed change and was assigned my seat in E+ and given a boarding pass. I knew the F cabin had 9+ seats for sale and asked if I would be placed on the standby list for UDU. I was told by the counter agent I had an open seat next to me and would have lots of room, but if I wanted to be on the list I could be. When I asked to be placed on the the list I was issued a Departure Management Card. I was upgraded 5 minutes before boarding, the only person on the DM list, on a flight delayed 3 hours. The plane left with about 12 empty seats in F. So you have to be patient, but you have to be explicitly clear with your request. If you are cleared and want to be upgraded, ask to be placed on the DM. There is no guarantee the system with catch you and the agent will process you.

At this stage, use this as a lesson and leave the 1K customer service people alone. They can't fix this and they aren't going to tell you the status/reason the other people were upgraded.