You can't rebook only a single leg of a two leg o/w ticket without repricing the entire ticket. What you are basically suggesting is that CGK-IST is booked in one booking class whereas IST-XXX is rebooked into another booking class. That is not going to fly.

You can try to ask upon arrival at IST but I would prepare for a "No" as such repricing of the ticket would be complex. If might be cheaper (and more simple) to purchase a separat IST-XXX ticket.

Unless your departure date is imminent, I would wait for a possible schedule change or cancellation which will allow you to rebook as you please.