From my reading on WW1, pre-war French emphasis was focused upon maneuver warfare and, as a consequence, great attention was paid to the excellent, highly mobile and modern rapid-fire 'soixante-neuf'. Very large numbers of this 75mm gun were produced to outfit the army. However, relatively little attention was paid to medium and heavy guns and howitzers. By 1915, the lack of such weapons at the front to counter the much more numerous large caliber German artillery assumed crisis proportions.
Steps taken by the French to redress the problem were two-fold: first, all modern medium/heavy artillery weapons that could be spared from the French fortress network were withdrawn and converted for field use; second, great numbers of obsolete medium/heavy artillery of the 'deBange' system were taken from reserve stocks and made as suitable as possible for use by the army. Fortunately, large stocks of ammunition for these obsolete guns were still available in the arsenals.
These obsolete weapons, while serviceable, were usually out-ranged by the more modrn German guns. They also lacked on-carriage recuperators to absorb recoil, requiring the gun to be re-laid upon its target after every discharge; this reduced affective rate of fire to about one-half that of their German adversaries. A large proportion of the French artillery park at Verdun consisted of such elderly types.
Fortunately, French industry was able to design and deliver ample numbers of modern medium/heavy artillery weapons before the old 'deBange' guns had become completely worn out. My GUESS is that the transition would have been sometime in mid- to late 1917. French artillery from that point improved greatly in power and gained a reputation for very high effectiveness.
Hope this helps.
B