I don't know. If something happens while you are in the middle of a trip somewhere and you have to spend more time at your destination, but you are uncertain about when you will be able to leave, what would they try to charge? The difference between your discounted fare class and full Y for the return portion of the ticket? Then, you could turn around and refund the upcharge because the upcharged portion would be refundable (you could not refund what you had originally paid for the segment, of course).