What do you propose that China can do after the regime change?
Afghanistan, Iraq and Libya and great examples that you cannot just throw away a dictatorship. What follows is practically a game of thrones of powers in the region which is a lose-lose situation for everyone, specially people of the "free'd" country and the neighbors.
I think it's wrong to compare North Korea to those countries. For one, the post-regime change problems in those countries have been caused by Islamic insurgencies, which is not something I'd expect to happen in NK.
Are you forgetting the Viet Cong so quickly?
Also, the Viet Cong were supported by a state, North Vietnam. They also weren't able to defeat South Vietnam; that took a withdrawal of US support and the actions of the regular North Vietnamese Army.
Best case would likely be China just annexing the whole country and imposing martial law for the next ~30 years. At least people would be more likely to get fed, and the nukes would be safe-ish.