On a flight from Frankfurt to Dallas, my friend and I got some pretty good prices on seats and then upgraded to business class. Being the miles monger he has been due to his love for flying, AND the fact that he ALWAYS asks about the possibility of being upgraded due to his status, the FA was extremely amazed that he flew in 1st class (when he purchased an economy ticket) because of him asking for an upgrade. It was a quiet flight for me in the (peon) Business Class seat.

At least on AA, yes, the FA's know.