Tapu Koko Hidden Ability?

Hey guys, so I have a Tapu Koko and I want to change it's normal ability to it's hidden ability, but when I use a dream patch it says "it won't have any effect". Not sure if the Tapu's are a special case with their HA or not. I'd rather not catch one with a dream ball or try to dexnav one, but I will do that if need be. Just wanted to see if anyone else experienced this or know of a way around it, thanks!