Originally Posted by
jetta2.0t
Except when the ticket goes through the clearninghouse (term from the paper ticket days), the airline is paid the face value of the ticket, not the class of service booked.
When you are upgraded (at least internationally) you are re-ticketed. That is why I got Biz class mileage both on the flight I mentioned and on a recent bump JFK-TLV.
If the operating airline has a valid reason to upgrade a passenger from the airline who ticketed it (and overselling would be a valid reason) then, I suspect, they charge the ticketing airline for the new, more expensive, ticket.