I purchased airfare in May 2020 for February 2021 travel, which I cancelled in December 2020 for FFC. I am now planning a trip on UA metal, but the fare is lower so I would forfeit the residual. I assume I can't just call UA and ask them to change the FFC to an ETC, but is there some way to accomplish a similar thing? For instance, can I use my FFC and some dollars to book a more expensive flight, cancel that to receive an ETC, and then apply a portion of that ETC to the trip that I am planning, so that I have the residual to spend later?