You changed/rebooked the date and the same booking class was available for rebooking, hence you only paid the change fee. You thought that the booking class would change without payment of up-fare? Just because you don't see a booking class online/when making a new booking, it doesn't mean that it is not available for rebooking (apparently).

You can't make non-refundable fare refundable by rebooking into a higher refundable booking class. TK is not that stupid.