I'm flying LHR-DEN-SFO on on a business class award on UA metal on Friday with a long layover in DEN. I no longer have UA status. Simple question: if I pay for a SDC to improve my connection at T-24 do I have to pay again if a better SDC (e.g. direct) opens up nearer to departure and I want to take it?

Sorry if this has been answered, my search-fu has failed me.