With Southwest I wouldn't count on it. I booked a flight on a voucher from a flight I had previously changed. The new flight I booked was cancelled due to the 737-MAX issues and rebooked the next day. Then the rebooked flight was cancelled due to maintenance issues, and they didn't have any way to get us to our destination until 12 hours before we had to return home. Southwest wasn't sympathetic to this situation and would do nothing for us, even though it was completely their fault. They also wouldn't refund the ticket because it was booked on a voucher.