I have an existing award book on TK online. TAS-IST-JFK. I need to change the origen from TAS to DYU. I see plenty of seats online for DYU-IST, but I am unable to change anything online. The call center only wants to search DYU-JFK which, not surprisingly shows no seats. How do I get TK to change just the first segment? Or even drop the first segment, and I'll pay for the DYU-IST portion?