There have been one or two reports of Qantas contacting customers one or two days before a flight and offering confirmed paid upgrades. But this is rare and almost certainly only due to extreme WHY overbookings. There are NO reports of people being able to action paid upgrades at check-in or on-board.

FWIW, I was contacted in March, 2 days before a flight, and offered a confirmed QFF Point upgrade to J from an O class SYD-LAX booking.