Last month I had to make a change to a full Y that was orginally booked with the discount. A few days before I had to change it and I could not make the change online. I too had to pay the difference as I was talking to a real person. The person on the other end of the phone tried to over ride it and even contacted the on-line people to find out how to get around it, no luck. Was able to change it to a non-refundable and save the company some money in the end.