Maybe China can move all the NK people into those hundreds of empty cities they have. Then turn NK into one gigantic paddy field.
That is exactly why they prop up NK - they don't want all the refugees flooding into China.
If NK overran Seuol, it would be a real test to see if the US would respond immediately, and that would make for some interesting US/China discussions. China does't want the US in NK, but I doubt SK/US wants China in NK either.