For international flights processed at the gate for an E or SE, they assume you are in the MLL and often will page you there to exchange boarding passes and attach the upgrade cert a few minutes before calling the flight for boarding. Domestic and Transbordere are done at the gate and if you are not there when they call names, but they knwo you've checked in, you will be caught as you board when your boarding pass is scanned. Sometimes it may be done when you're on board and already in your seat.