Earlier this year I ran into this situation. Flight was completely sold out in Y, but most of F was open. Since we were unable to secure seat assignments in Y, we 3 Golds went to the airport early. Checked in about 3:20 prior. While checking in, agent was able to secure us 1 F seat, then the system locked her out. Other two were told to standby for seats at gate and waitlisted for upgrades. Finally called to podium for 2 Y seats, I asked what happened. Seems that everyone else had upgraded ahead of us, including several non-revs. I took my seat in F, sent kids back to Y. Wonderful gate agent boarded, yanked two non-revs from F and went back to tell my girls they had a "special secret assignment" in F. This was a very special gate agent! Don't think this will be an issue with you, unless you are more than 1 person on the itinerary.. BUT if you check in prior to 3 hr EUA, and agent is unable to upgrade you, I would advise to decline checkin until the EUA has run. The angel gate agent told me that nobody even checked to see if there WAS a waitlist, assuming that all Elites had gone up during EUA, and the remaining seats were assigned to companions and non-revs at gate.

Robin