Originally Posted by
freeagent
If there are empty seats in First at check in and the upgrade list has been cleared, there will be an option to buy an upgrade at check in.
This.
The last sensible thing to do is to sell UG's via sticker and then do them for free for the same people.
OPUP's to prevent an overbooked Y cabin from oversell are a different story, but that's the case on international flights where there are no freebies either.