In March, I received a double UPG on ORD-LHR where I had an M fare with e-VP to confirmed C . I was in The FL and asked the source of this largesse. Reply: A full coach and J section so they were moving up pax based upon fares. Same thing happened on a JFK-SFO last month. I got the last J seat on an EXP UPG of (I believe) a K fare (the special SFO-JFK-SFO OW fares,) and the gate agent walked me to the seat, to move that pax to FC. Asked her on the way if I possibly would get the FC seat. Her reply, the pax in that seat paid a higher fare basis than mine. Very logical behavior. So it can't be merely status and time of request.