This is currently a hypothetical question (although the answer may affect what tickets I actually buy) so I will give a made-up example (although the fare amounts are broadly 'right' for some of the routes I've looked at).

Imagine a route XXX-AMS-YYY, for which a KLM MFFxx fare is $1,500, ZFFxx is $2,500 and JFFxx is $6,000 (where 'xx' is the relevant country code, of course). So, I purchase a ticket in ZFFxx (cheapest business, but still fully flexible).

Then, later on, I need to change my travel dates. I contact KLM, and they say that there is no penalty fee to change the date (as it's a fully flex ticket), but availability on the day(s) I want to travel is J9 C0 D0 I0 Z0 - so if I want to change, I need to pay the fare difference between Z and J, which is $3,500. This is more than the ZFFxx ticket cost in the first place, and is more than I (or whoever is funding the ticket) is willing to pay.

However, it is quite likely that there is still space in the economy cabin - eg: Y9 B9 M9. So, if I ask KLM to downgrade my booking into, say, M, what will they say:
a) That is not possible?
b) That is possible, but we will not refund the fare difference?
c) That is possible, and we will refund the fare difference?

I can't find this addressed in the ZFF fare conditions anywhere, nor in the Conditions of Carriage (which mention involuntary downgrades at point of boarding in the event of overbooking, but don't seem to mention downgrades 'requested' in advance due to lack of availability). So I'd be interested to know if it is addressed anywhere else, or if anyone has experience of such a situation?

I guess the other option would be to cancel the ZFFxx booking (as it is fully flexible) and rebook in MFFxx - but this is far from ideal. (It seems that cancelling even a 'fully flexible' booking incurs a processing fee these days; and there could be additional complications if seats are no longer available on some of the (other) flights in the booking, if the base fares have changed in the interim, or if money needs to be refunded to a credit card before it can be spent again. It also wouldn't work if some sectors had already been flown - eg: the outbound sectors had been flown, but I needed to change the inbound...)