You def. can't make such rebooking online (you are changing origin/routing, carriers etc). I assume that this is a Star Alliance award ticket? You would need to call TK CS or contact your local TK ticket office to try to get rebooked, but be prepared that it may not be as smooth as you hope. Changing an award to new origin/new routing and carriers etc is challenging and you need to have award availability for all segments, so good luck!