For SEA-ANC in paid F last month I got an email about 8 days prior to the flight stating that they they were looking for volunteers to take a different flight. It turns out that they had subbed the aircraft to one with fewer F seats and were oversold. I called and they offered a $350 voucher for switching to SEA-JNU-ANC getting me in to ANC about 2 hours later than my original flight. Not bad... I got to see JNU for the first time and the view of the coast was spectacular.