"What is always uncertain is what is the reprice that will be applied. And no one can know ahead of time on any airline."

In BA's defence, that's not quite true, at least as far as knowing the procedure which will apply to establish the new fare. I usually find BA.com quite clear on the re-pricing conditions for changes & cancellations in the blurb at time of booking. Assuming we're talking about restricted fare categories which do allow changes, and if you look, it will either specify re-pricing at whatever equal or higher fare is available for the new flight and date, which is usually the case for changes made before the first sector is flown, or for mid-journey changes made after the first sector, re-pricing will often be to the historic fare at time of first booking (in other words possibly no fare difference if the same fare category is available). This is of course in addition to any admin fee to do the change, the "change fee".

Where I do find BA.com bit less user-friendly to the uninitiated is when it specifies a "change fee" in one paragraph and then a bit further down in the small print you find (at the end of a different condition & without sentence break) "changes at any time changes not permitted." Not well written but has been like that for years!