this does happen on AA. The friday before Christmas last year, almost missing the flight because of traffic issues, running to the gate , I hear my name called just after exiting security. When i got to the gate, the GA informed me that she was upgrading me because they were oversold . i had requested a sticker upgrade online, being platinum, but she said that this upgrade would be free. She even paged another passenger so that my partner and I could sit together . he was in paid F. the sticker upgrade never did come through for me. it was only lax-sfo but a nice gesture nonetheless.
A nice holiday present from AA.