It seems to me that this current security crisis in Korea is a golden opportunity to begin reuniting the Korean Peninsula. I believe China knows that would be in their best interest. If Korea is reunified as what happened to Germany, then the first thing that will happen is the Americans will leave! Isn't that what China wants? A prosperous demilitarized Korean neighbor would compliment China nicely---both nations will prosper and regional tensions would ease.
North Korea is so bellicose and outside diplomatic norms now that even China can no longer tolerate the snot nosed brat in its backyard who is continually disturbing the peace and acting like a yobboish lout! I believe they now want to send that young man to his room and take his X-Box and rice bowl away. Even China has limits to what they will tolerate in their backyard.
If the Norko's lob something, then our response will be proportional. I don't see regime change happening. North Korea can't sustain it's foul ways. They will crumble from within.