Turkish Airlines Primefly Ticket Change Costs Double

I have a Primefly ticket with reservation number R43GK3 for a flight departing on January 16 and returning on January 31. The only reason I chose Primefly was because of the 'Free Change Option' you offered at the time of purchase. To have flexibility, I paid a significant difference and bought the Primefly ticket for ₺6,679.39 in September.

Today, when I tried to change the ticket to depart on December 23 and return on December 31, I was informed that I need to pay an additional ₺6,408.84. However, when I try to purchase the same ticket from scratch, the price is ₺9,939.23. The actual difference should be 9,939.23 - 6,679.39 = ₺3,259.84.

When I called the call center, they mentioned that the ₺6,408.84 is a 'Class Difference Fee'. I calculated the difference between the two tickets above, and there was no explanation for the extra ₺3,200 you are trying to charge. The taxes and fuel charges for the flights are the same, and the difference between the tickets is clear, yet the requested price is ₺6,408.84 instead of ₺3,259.84.

I paid the Primefly fee for the free change option, and now you are asking for almost the price of a full ticket again. I always buy Primefly tickets because we need flexibility, but I see that the feature we want to use is useless and is just a way to charge us extra money. If I had known this earlier, I could have bought Extrafly or Ecofly, as you are asking for a ticket price again when I want to make a change.

We are very loyal Turkish Airlines customers and fly very frequently. We buy multiple tickets in advance and always choose Primefly. Unfortunately, I see that this is very misleading and feel deceived.

What I expect from you is to first explain mathematically why you are trying to charge me double as I calculated above. Then, I expect a refund for the difference we paid for Primefly. Our trust in Turkish Airlines is significantly shaken.

Thank you in advance for your prompt response.