I checked in at 24 hours. However, when I contacted the AA agent, she told me that it was based on who arrived at the airport. From reading all of these threads, it is hard to tell whether it is checkin time or arrival time that determines who gets the upgrade (on the same tier). When I told her that I was going to checkin online she said that had nothing to do w/ it... but then I asked how they would even know if I was at the airport if I had already checked in. She was didn't really have an answer?? I wish they would just state the rules and then follow them, consistently! Should I keep calling the automated upgrade line or is one time sufficient?