I got the same email the other day flying AUS - SEA. They said the flight was oversold and offered only to pay the difference between coach and U fare. It was a 737-800 on the original booking and did not change. There were 4 seats open on the flight, all middle seats in the back of the bus. I did not get booted but would have been furious if they did it to me. This was not an aircraft change - and I do not get why they oversell.