Gee, I wish UA would offer compensation when I get downgraded...but since the two instances so far were ones where I hadn't yet given them the upgrade certificates, I suppose the certificate for a future upgrade was reasonable profit for me.

Sidneyjb, I would have taken your offer, too (as well as those of the other posters), but in your case I might have negotiated "Can I sit up front other than take-off and landing?! " I'm pretty sure the answer would be no - they'd need a place for the FA to sit in case of turbulance during the flight, too.