Moving from 21J and 21L to 20J and 20L worked out well for us. There were 36 or so people on the standby list. 12 ended up in Polaris, the rest were assigned the empty aisles, windows and all but 10 middle seats in E+. Fortunately, one of those empty seats was 20K. So, my daughter and I got to enjoy the extra room.
From what I could tell by looking at the seat map right before the GA started processing standbys, the passenger in 21K never moved. I enjoyed all the speculations. But I am leaning towards the theory that she upgraded to E+ and chose the less expensive seat.