I experienced 2 very different scenarios on a trip down to CUN last week. I was booked in PY lowest for a roundtrip YVR-CUN and requested eUp to J. Checking expertflyer for AC932 on the way there, I could see there was lots of space left open and to my surprise, my eUp actually cleared before T-24h.

The way home was a different story. Checking the seat map for the flight home on AC933 it looked like there were 13 seats open in J (I guess there could be some people who hadn't selected a seat yet). I checked in right at the T-24h mark (I was sequence #2) and when I looked at the eUp list, there were 26/30 booked. As the day went on, however, that number kept climbing and the next morning J was listed as full. The whole time I was #1 on the eUp list and I was optimistic but somehow that number kept climbing. I'm curious to know if people get eUpgrades cleared at check in and never show up on the upgrade list. Can someone please enlighten me? Thanks in advance!