It has been reported a few times in other threads that since the big change of TK M/S earlier this year, TK agents have been more restrictive in allowing a change of origin or destination following an invol. schedule change of more than 15min. In the past it was def. possible (within same region of course) but now it depends on the agent that you talk to.
Try to contact your local TK ticket office for assistance. Emails will often do as calling these offices are in general not possible.