I saw someone try this a few years ago on a flight. I believe it was HNL-SFO on UA. I was in F, and the two F seats behind me were empty. A guy snuck up from coach part way through the flight, nabbed a seat and immediately dozed off. An FA noticed almost immediately, came and woke the guy up and asked if he was in the correct seat. He readily admitted that it was not his seat and he'd upgraded himself. He slunk off to coach without any fuss.

It was like the classic Seinfeld episode when Elaine snuck into an F seat.