In that situation,. there's no need to rebook the flight for another day just because you need to cancel today's flight. That is, when you tell them you're not flying today, you don't need to rebook immediately - you can just tell them to cancel your reservation and check-in for today's flight and then leave the value on the ticket but not associated with any particular flight. Then, when you you know when you will need to fly, you can then apply the ticket to that flight. This eliminates the need of making a speculative booking that you probably won't need to fly just to have one on the ticket. Of course, with this method, there's no opportunity to use the lounge on the new date that you probably won't actually fly.