Chang Ge Xing is actually both seinen and shoujo. Seeing as how seinen and shoujo just denote the demographics that the magazines the comics were originally published in are targeted to, and have absolutely nothing to do with genre, you go by that. It was originally published in a chinese magazine targeted towards young women, so it is technically a shoujo. However it was serialized for Japanese audiences in ultra jump, a seinen magazine, so it's also seinen. It's both.