Recently I was flying from JFK-HKG on the Sunday nonstop. When I was leaving the British airways lounge they asked if I was waiting to be downgraded? I explained that no....I was full fare business class. They explained that they were offering $3500 USD per person to downgrade one way and a row of seats in Economy for the long haul flight (at least 3 seats) and a duty free coupon for onboard spending for $250. The funny thing is that the woman who was asking people to downgrade at check in was only asking women and by the time I got on the list (at the club) I was the only male and #7.....They ended up "downgrading" 5 people...all women.

So in answer to your question....They downgrade and pay you big for it.