Thanks! I'd forgotten about that thread. However, it seems to be referring to an AF fare - and the KL/ZFF fare rules I'm looking at don't seem to include the long list of "REPRICE USING FARES IN EFFECT" options that the AF/SSFDK fare had...

Looking at some current AF and KL fare rules (for long haul) it seems that xSFxx (ie: Semi-Flexible) fares include conditions (under 'Penalties') saying that if a change is made:
- In the same booking class, then the new fare can be lower, equal or higher than the old fare;
- In a different booking class, then the new fare must be equal or higher than the old fare (which basically rules out fare-class downgrades, unless the fares have risen dramatically in the interim)

However, these conditions are not mentioned in the xFFxx (ie: Fully Flexible) fare rules - and my (limited) understanding of fare rules is that if something is not mentioned, then it does not apply (eg: all fares are flexible by default, unless the fare rules say they're not). This could be taken as implying that a fully flexible fare can be downgraded to another fully flexible fare (as there are no conditions saying the new fare has to be higher) - but I'd like to have something a bit more robust than this, before I try to rely on it...