The "credit" is basically just the unapplied coupon for your original trip. If you want to do anything with that coupon, you have to change that trip, which involves changing all segments of the trip, including those already flown. The new travel will reprice, including repricing already-flown segments, as necessary so that all segments keep a valid fare. Generally, if you change a ticket after departure you are bound to use the fares in effect on the original ticketing date, and must complete travel within one year of then.

United has mucked a lot of this up with their "FFC" concept, which I haven't paid close attention to, so some of the finer details become less clear. Once you change your ticket, you should be bound by the rules of the fare(s) on the newly reissued ticket, but note this includes those fares used for already-flown segments.