I've never seen such a drastic name change, but definitely some name changes. I have a lot of French Canadians in my tree who moved to the states, and they frequently changed things like Jacques to Jack or John, Oliva to Oliver, Marie to Mary, Juliette to Juliet, etc. It is the big melting pot, after all!
Is it possible that Oliver or Oliva was his middle name in Quebec? Because normally I'd expect a Francois-Xavier to go with Francis in the US. It's a bit odd to change so much.