Originally Posted by WineCountryUA

If you cancel/ change a flight that had a waitlisted or cleared mileage upgrade --- the miles (and, if any, copay) will be refunded with no penalty. This should be automatic but occasionally you may need to call the MileagePlus Service Center if not done.

If you want to place a new request on the new flight, you will have to do that separately. If using an agent, they can do both at the same time.