I've never done an inflight switch: from reading here, I've gathered that it's considered poor etiquette at best and might not be allowed at all by the FA.

If my wife and I are traveling and we get the single last available upgrade, she takes it... If it were my mother or father, I'd let them take the better seat too. I generally recover from jetlag faster than others in my family - they'd get more value out of the sleep.