If you book a trip and pay with a voucher and then cancel, the refund is to a new voucher with the same expiration date as the one you used. If you book a trip for a date beyond the expiration date of the voucher and cancel it after the expiration date, my strong assumption is that there is no refund. I don't know what happens if you keep changing it to dates in the future. If you are able to, that means you can keep the value of the voucher indefinitely, which seems like a big loophole, but I'd ask an agent to be sure.