To my mind this comes down to how delayed you are at your destination: in this case it's DEN, so if they can't get you to DEN within 2 hours of when you'd booked without forcing you to leave earlier than you initially booked, you should be able to request a refund. As MSPeconomist also pointed out, if they changed the operating carrier on you, and cannot get you back to the originally booked operating carrier without putting you more than 2 hours late, you should also be entitled to a refund. I hope this helps.