EU4 Milan - Italy - Roman Empire - Beginner's Guide - Tutorial You can change your countries religion via the Console Command Tab. So as far as I know, the only way to convert would be by event or decision. Shinto has no religions it can convert to in fact, most religions don't. I don't know if there's an event where you can change the state religion too, but it's a start.

Problem is getting those in the first place I think once the western powers start showing up in Japan there's a chance of Japan's provinces converting to Catholic. If you want to change to a completely different religon you could wait for some religious zealouts a kind of rebels to show up and give in to your demands, they usually want you to convert. If you do swap religion definitely make sure you convert the other provinces you have or you will have rebel zealots springing up all over in non-state religion regions. Is it possible to convert my countries religion? Like if I wanted to change Japan from Shinto to Catholic? Then you can go to the religion tab on your nation's interface and one of the buttons there will change religion.

Crusader kings 2 religion id