Modifying country sprites

[Sybreal]

I'm wondering if there is any way to modify some files to make it so that a country can use a different set of sprites. Like a fully westernized japan using the latin unit sprites instead of the Chinese sprites. It could just be me but my game does not update sprites when westernizing. If it shouldn't do that then that is just dumb... Napoleonic squares shouldn't be wearing straw hats.

Also, I was wondering what programs I would need and what files to modify to create my own custom sprites if that were possible.

Thanks.