Same process to use the residual value on the next ticket. When you use FFC that doesn't cover the entire ticket cost, UA will charge the rest to your card. No need to call UA; in most cases, it can all be done online. I did something similar last night -- I had one trip that I no longer needed to take, and I used the resulting credit toward two other, less expensive trips that I did need to take. All online, no phone calls.