OK - just wait it out and keep monitoring availability/hope that seats will come up for sale (it may not be until very shortly before traveling if this is a big travel date)

One of the reasons that you can't rebook a segment only is the 'married segment' issue, i.e. TK will not allow you circumvent married segment limitation by allowing to rebook only a segment. Example: say LHR-IST-HKT is in high demand and TK only sells this itinerary in a high/expensive booking class. Then it is not the intention or allowed that pax book the cheaper MAN-IST-HKT and later rebook the MAN-IST segment only. Then the entire itinerary needs to be rebooked (and repriced).