Originally Posted by Steve M

Were you logged into your MileagePlus account at the time you booked the ticket? If not, then the 24-hour refund policy does not apply.

Also, there's no need to turn the ticket into a travel voucher. Simply cancel the reservation so that you don't no-show and keep the record locator. The record locator will retain the entire value of your original ticket, which upon payment of a $150 service fee, can be applied to future travel. There's no need to pick new travel dates, get a travel voucher, or pay the $150 until you know what your new plans are.