If we are talking Open and Closed Stances, taking one step from running is an easy way to do it. Like, you run, and then stop. If you are in closed stance, you'll find yourself in Open, and vice-versa. There's another way, but I don't know how.
I also think there should be a stance switch in DOA6, like Jacky and Jann Lee's for every character, by pressing H+P+K, so that you don't just jab when you press it anymore. That'd be a cool addition.