How widespread are they? One on each border or just sporadic?
I'm not sure there's an official rule, but usually you see them systematically when the border is on the summit of a col. There are some stones on flat lands, but they are more sporadic then.
And what happens when departement borders move; do they move/alter the stone or just leave it there, with outdated names or in a position that's no longer relevant?
Borders never move, don't you know?
More seriously, I have seen only one example of such a stone at a no longer relevant place: the
Col du Hantz used to be on the French-German border a long time ago, but the stone is still in place.
A