We recently needed to cancel the return flight. We called United, and they said we would have a future flight credit.
A couple week later we called to book a flight with the credit. First of all, they said that it need to be priced as the return of a roundtrip, so we couldn't use it for a flight leaving the same city which the original flight departed from. It also took a long time, because they wouldn't tell us a dollar amount how much the FFC is worth. It depended on how the system would price out the original flight that we took and the new flight as a roundtrip, which took very long. (not sure why it was booked as roundtrip, would have been same price to book separate 1 ways and would have save us some headaches)
We were also told that because the ticket was originally booked using an online travel agency, we would need to pay $50 extra on top of the fare difference. Figured that this would be something people might want to know about. Booking through Priceline has the benefit of the easy cancellation during the first 24 hours, but if you will need to cancel the ticket later and get a future flight credit, it would be better if the ticket is booked directly with United. If you will need to cancel and get a FFC, you will lose $50 if you book using trav agency
Also, when we did change the ticket and paid the fare difference for our new flights, we were told that we do not have the option to back out within 24 hours and get a refund for the payment. The 24 hour rule is only for the the original booking...