Originally Posted by
DALFA
Not on international flights. An upgrade must be done over 24 hours before departure, the only time someone is upgraded to business from coach at the gate or at the counter is if coach is oversold and that person gets involuntarily upgraded or if he/she decides to pay for a full fare business ticket at the gate ($3,000+).
Not true anymore - Platinum and Diamond medallions can standby to upgrade with a SWU if they have a Y,B or M ticket.
From the list of Medallion Benefits:-
Day of Departure:
If a Medallion member and companion (if applicable) do not clear the waitlist prior to departure, they will be automatically added to the airport standby list at time of check-in and will be cleared at the gate if space is available.
I have used this coming out of NRT.