I have wondered about doing this also. Was the up-fare significant? I often buy months in advance and have pretty cheap tickets and low priority upgrade class tickets. I have never called to ask about buying up to Y, S, B, M, H or Z to up my chances of getting an upgrade. I have searched for the refundable coach but the cost is often more than I class.
As an example, I have a K class ticket. I want to try to upfare to H, so I get an instant upgrade at 120 hours. Can they do that and what has been your experience with costs?
Thanks