I've had it happen a couple of times. ORD -> DEN on an old config 777 and the best yet was SYD -> SFO from C->F. I got Y -> C using an SWU and due to a mechanical canceling my original flight (SYD -> LAX) they put me in F (1k for this 1k). What a treat!

I've also had a GA upgrade my wife (we were flying together, I got the UDU, she was on a Y award) into C so we could sit together. This was from DEN -> SFO.