A couple of years ago, I was upgraded at the gate from SYD-SIN from Y to J. Was travelling on a cheap web fare and flying stand-by. I had been cleared onto the flight at the check-in counter about 45 mins prior to departure. When I got to the gate, I heard my name being announced and was afraid they were going to take my seat back, but no ... they traded by Economy Class boarding pass for a Raffles Class boarding pass.
Was a UA *G at the time!