The thing is, when the same booking class was obviously available, why was it not shown in the screenshot above? Instead, two other booking classes were presented, but the change fee was calculated based on the (hidden) original booking class. Quite confusing and honestly not what I would expect to see as a user. Just before payment, it was clearly shown that I would be changing to a flight in a different booking class. Usually, I would expect to get what I see on the screen when paying for something online.

Being able to change the flight to itself and having it reissued with email confirmation is also quite bizarre IMHO. I mean, a simple message like "In order to change your flight, you need to select a flight and booking class that are different from your original booking" would be more appropriate.

TK might not be so stupid, yes, but their user interface is not the most intelligent.