Rather than rely on a "glitchy" currency conversion during payment, why not force the charge to be quoted in the appropriate currency by just changing the AF/KL website to the US version (to pay in USD) or the GB version (to pay in GBP), whereby the amounts to be paid for SAF will naturally be quoted in the "local" currency?

The "glitch" referred to earlier shouldn't occur; despite the explanation offered, I think it's tied to the process of changing the currency at the time of payment, rather than being any inherent inability to charge in a different currency than the ticket was charged in. By going to the country version of the website (either AF or KL) using the currency you wish to pay in, you avoid the "glitchy" payment-time conversion and get a proper, smooth quote in the required currency - no conversion or trickery needed.

Extra options (unlike upgrades) do not have to be purchased from the same country as the original ticket, and you do not even have to purchase from the same airline that sold you the ticket in the first place (you can go to AF even if you originally bought from KL, and vice versa)

Here are the prices charged by the KLM (US, AU, GB) and AF (PF) websites in, respectively, US$, AU$, GB£ and XPF, for my upcoming KLM flight (bought in Euros on the Irish website):