I've seen it happen on an IAH - ONT flight. It didn't happen to me (not elite) but it happened to the guy sitting next to me. I was paid F in 1B. A pilot for another airline who was commuting sat down in 1A. About ten minutes later, the GA explained that unfortunately she had to rescind his upgrade because he was a silver, and a platinum showed up at the gate last minute.

Of course, this was after three gate changes (from one side of IAH to the other and back) and an aircraft swap, so there were some additional circumstances. But, the silver understood and the plat took the seat, no big deal.