About one year ago,

I had an intra European KLM return on a standard fare.

I went on the to the “modify my flight” tab and modified the booking. Kept the same itinerary but modified it to Flex.

One week later I cancelled the booking and KLM refused to fully reimbursed me, saying the ticket was originally in standard.

I contacted a lawyer and contacted KLM together with them saying I was starting the process to sue them (or take the ticket office in the country of sales to court) for not respecting the conditions of the new ticket (or contract of carriage) that “cancelled and replaced” the older one.

The lawyer forwarded the letter to the head office in Amsterdam on Monday (or by email I can’t remember).

On Thursday I was fully reimbursed.

The terms and conditions of carriage were vague enough back then, I remember the old terms leaving us all with doubts before taking action.

One airline that I know is much more clearer on this. (Changing from standard to flex) is ICELANDAIR. Which only reimburses the added amount to the standard fare. But it’s clearly stated on their website.