it might help to think of it thus:

If you're buying a Light fare, the change fee (which was: €50 for "short haul" (aka AF domestic), €70 for "medium haul") is now replaced by an amount equal to whatever amount was paid as the pure ticket fare of the ticket you wish to change (plus the sustainable aviation fuel charge that is compulsorily added to the price, if applicable) - so, the total price paid less all taxes.

And rather than "changing" the ticket via the payment of that €50/€70 fee on top of a fare difference, the new procedure is that you request a refund of your Light ticket - in advance of travel - via http://refunds.klm.com ("Voluntary cancellation of the ticket (as per conditions)") which leads to the taxes being refunded to the original payment method. And then you just go and book a new ticket.