One interesting aspect is the Flex option that you purchased. You basically bought additional flexibility on top of an award which has static flexibility rules. And Transavia is the one that sold you that package I assume?

Would you have been able to get a refund with the standard rules for these Transavia award tickets? If you are making use of the Flex option you would need to, I expect, talk to transavia (Assuming you paid them for the flex option).

Was the ticket issued by FB (057 prefix on the ticket number), or by transavia? I'd imagine that only the party that issued the ticket could actually make the change in practice. Of course if the ticket was issued by FB, and Transavia sold you a flexibility package that FB does not honor.. Well, as you found, that may be problematic.