And as part of the gate upgrade procedures, the GA's should move the downgraded passenger to their originally-assigned seat, and force the standby passenger to move.

I always take upgrades as a gift horse and nothing more than that. And I understand that sometimes a gate upgrade is cleared "prematurely." But please do not "punish" the passenger twice by making him/her think he/she was upgraded, and then saying "Sorry.....you'll have to sit in 28B instead." It's terrible customer service.

Disclaimer: This of course assumes that the downgrade was legit, and no shenanigans came into play. This particular scenario sounds "legit," since I doubt that the GA would have assigned a standby passenger to the OP's seat if the intention was to put a friend in the F seat.