This phenomenon is called a "Jespersen's cycle" according to Wiktionary.

https://fr.wiktionary.org/wiki/cycle_de_Jespersen

Also according to Wiktionary, "guère" is unrelated to "guerre" (war) and "ne ... guère" is therefore not the product of a Jespersen's cycle.

https://fr.wiktionary.org/wiki/gu%C3%A8re