Now that Alola has introduced regional variations, this opens some new questions:

How does a Pokemon's type change?

When I think of "regional variation" what comes to mind are adaptations changing the physical appearance of the Pokemon. So maybe a Vulpix and Ninetales with extra fur to protect from the cold, maybe the fur becomes white due to camouflage, and even through all this it would still be a fire type. That's how my headcanon works anyways.