Many airlines have introduced fare brands, like AF light, standard and flex on AF.
Those 3 brands can book into different fare buckets, like Y, B, M, etc for Flex. Even those can have different fare basis like K********.

I do not know how AF IT works when changing dates on a ticket, but it is likely that you retain the same fare bucket, possibly the same fare basis.
New Flex tickets might be issued later at at a lower fare bucket and therefore less expensive. I doubt that changing the return trip on the originally-issued ticket would search for the lowest fare bucket available.
Just a guess for AF.