China and North Korea have a military mutual aid treaty (Sino-North Korean Mutual Aid and Cooperation Friendship Treaty) which was signed in 1961 and renewed over the last several decades. It runs until 2021.
China may not be interested in regime change if it results in a democratically elected government that supports reunification with the south. They might be willing to go along with leadership change that brings a different, less crazy strongman into power.
“They might be willing to go along with leadership change that brings a different, less crazy strongman into power.”
That parallels my thinking. China could install a stable dictatorship of insular, hardline military oligarchs, who would abstain from developing nuclear weapons and delivery systems, and generally tone it down with the South. If North Korea’s economy improved, it would only help their main trading partner, China. The US would tend to lose interest and justification for its presence; South Korea would forget why it ever wanted us there. Some might argue that China actually WANTS to keep us in Korea. `