Originally Posted by
Jdbailey710
... it's still doing the future flight credit when I attempt to re-book online rather than receive the travel certificate for the price difference. ...
FFC is the travel credit / cert.
UA will not permit a FFC to be used for the same trip in the same cabin. However, you may be able to use that FFC for a trip on a different day / flight and then cancel and use the newer FFC to book the preferred day / flight.
Yes it is awkward, but it is the workaround.