If you are searching in France for info prior to Andre Megny's birth in 1620 Isle De Ri. "dit Lagace" was added to Andre's name during his military career. It means "said the Cannoneer", and was an early way to identify people.
The name in France was simply Megny, and became Megnier fit Lagace' in Quebec, perhaps because Andre was illiterate and could not correct the record (?).