Just called twice. Got the same answer both times.
If its still within 24 hours of making the reservation, then one would simply cancel the reservation and re-book. Keep in mind that if the fare has increased, UA is unable to adjust the fare to match the old reservation.
If you're after the 24 hour period, nothing can be done if it is a refundable fare.
If it is a refundable fare (and its been after the 24 hour period), then one would simply cancel the reservation and re-book. Again, UA is unable to adjust the fare to match the old reservation if the fare has now increased.
Oh well.