Originally Posted by SK AAR

TK can rebook even if no award availability is present. The issue is if you can get the issuer of the ticket to go the extra mile for you and contact TK about rebooking options. Most FFP agents will just search for other flights with award seats available, so you may have to be persistant here. But the issue you are facing can indeed be complicated to resolve depending on how your FFP decides to act. TK can do nothing for you directly as you need to deal with your FFP/the issuer of the ticket in these situations (only in case of IRROPS TK can take over and do the rebooking)

As a last resort you leave the reservation as it is and let the check-in agents deal with it on the day of departure (check-in agents can't let you travel on an invalid itinerary so TK at PTY or IST will have to rebook you)