the operating carrier can accept an eticket coupon for one flight and use that coupon as payment to allow the passenger to travel on a different flight, even if the new flight violates the fare rules of the ticketing carrier. happens all the time for operational reasons and does not require the ticketing carrier to reissue or revalidate. convincing the operating carrier to do so if the challenging part. a good reason (am sick, flight delay, etc) helps.