OP - Welcome to FT. Some details will help provide you with better advice:

1. Did you purchase directly from TK, e.g., through TK's website or did you purchase through a TA, third-party-site or consolidator?
2. Exactly how long after you booked did you call in? (It really matters).
3. Do you have an email from TK confirming your original travel and providing an e-ticket receipt (not simply a confirmation), and is there anything from TK after you called in to make the change to which you did not agree (check your spam filter)?

Let's get the facts down first.