Trying to change an Y flight for a family member. Change is only for the outbound leg.

The change is more expensive than price for a new ticket on that leg + change fee. Explanation given by AF: even though I make no changes to the return, the return leg is also re-priced at today's fares. Result: although I only change the outbound where there is very little fare difference, the modification costs a lot because there has been a change of fare on the return leg, despite me not changing that.

Is that the way it's done? The full ticket is repriced even if you only touch one leg?