My experience is that 1) they should try and find another Business Class seat, 2) if none available, then try and upgrade the passenger to first class. If neither of these are possible then the purser should give the passenger a card to mail in to WHQ and usually they send a voucer for $150.

Now, if I'd actually paid $3,000 plus on a trip and coming from Asia to the US then I would have a little chat with customer services once I got home and would insist on the difference between C and Y for that segment.

But United's first response is to usually try and give a voucher for $150 and see if that settles the issue.

Billy