Originally Posted by WineCountryUA

Generally a FFC can not be used to book the same cabin/flight as the original ticket. The best self-help approach is find a different flight or different day for the same price (as the original or new booking or in between) , book that and then cancel and use this new FFC to book the original flight. Any decreases will be issued as a FFC for later use. In each case use the FFC as a payment method, not as a change flight.

Again coming in from GF may create limitations on usable credits, but that is a different issue.