Issue with THY Ticket Date - Seeking Understanding

I purchased a round-trip ticket from Ankara to Seoul. By mistake (due to the 13-hour flight duration causing confusion), I booked the return ticket for one day later. This mistake cost me an additional ₺6,000. I immediately contacted THY customer service and explained my error.

To switch to the ticket that is ₺6,000 cheaper, I was asked to pay an additional ₺9,000. Is the cost of a momentary human error really ₺15,000?

I am a teacher taking my students to South Korea. All my students are currently scheduled to return one day before me. They are all under 18 years old.

We see Turkish Airlines as a part of our homeland. I am expecting a reasonable offer in return.

I kindly request that my flight date be moved to one day earlier at the same time. It would be just changing from August 1 to July 31. The times remain the same. If needed, I can also share the photos of the July 31 flight tickets I purchased for my students.