Originally Posted by
Lemonwings
That's what I thought, but my example above seems to contradict this. I have the impression the difference was calculated with an implicit price for the new flight higher that the one displayed when simulating a new booking. (All tickets were flex but I haven't checked the exact booking classes.)
The price charged to a "new" customer must not always be the same as the price offered to an existing customer seeking to change an existing ticket.
There is never just one price for a flight, so checking what a customer would be charged today on a hypothetical booking is not a reliable guide for what fare difference may be applicable in your situation.
This is true for all airlines and is not AF/KL specific.
Unfortunately, people seem to think that buying a Flex ticket puts them in the most beneficial position possible, and that it allows them to change tickets without paying more than they otherwise might have to - it doesn't. I would always advise avoiding Flex tickets unless your intention is to cancel outright.
(If you have not yet commenced travel, I would advise you to check the price of the cheapest currently available ticket for the trip you now wish to fly, and see if it makes sense to cancel your Flex ticket and book a cheap inflexible ticket instead. Of course, if you still don't know what trip you actually will take, then wait until you know this.)