They could go all the way down to DMZ, if they travel 1,000 km from their base down to Korean Peninsula.
All GP's along DMZ may wake up one day to see PLA soldiers on the other side. Not a comforting thought.
Plus North Korea is not very densely populated, and the Chinese need space. I believe China also has made claims parts of Korea. They usually make a ridiculous but official claim to justify their imperialism.
I'm not so sure that it would be a bad thing. China created this mess in th first place...I think that they should be the ones to clean it up. China does a lot of business with the South Koreans and Hyundai is the most popular cars in China. Having been in china a few months ago, I can comfortably say that the average chinaman is none to happy about having Kim Jung Il doing stupid things not to far from them and they don't trust him to think that he wouldn't turn on them.
The Russians also are sending troops into the region to participate.