Aroujnd 3 years go, restricted Z fares within Europe had the go-show provision in the fare rules and I used it a few times back in the days. Now this has disappeared.
Regarding go-show (same day stand by on an earlier flight), you can indeed book a later cheaper flight and take an earlier flight that would be more expensive, in this case there is no fare difference that is chargeable (there needs to be seats available in the same or lower class). Since go show is only allowed on FLEX fares nowadays, there is obviously no change fee either. Note that the flight needs to be exactly the same, although perhaps a CDG AMS XXX could be changed for an earlier direct AF CDG XXX .