You have become victim of a deceptive and probably illegal practice. For new bookings, K is available. For rebooking, dynamic personalised availability kicks in. K is zeroed out forcing you to pay the upfare. Since the fare conditions and website describe a rebooking against "only" the rebooking fee without fare difference, since K is indeed available, this is probably illegal and if done with intent of financial gain potentially criminally actionnable. Good luck fighting it in court. I hope somebody does a criminal fraud complaint. It's bad stuff.

I usually just buy a new ticket, often on another Airline, and get my taxes back. Once I could outsmart the system with a new reservation and PNR having the old ticket applied to it.