There's a good possibility that even if regime change happens the North Korean citizens won't go quietly. Many hate the regime but many are also brainwashed into loving it. Worst I can see is Vietcong-level resistance to a democratic North Korea or unified Korea.
As things were understood about Nazi Germany on the brink of defeat around the time of the failure of the German offensive known as the Battle of the Bulge:
There's a good possibility that even if regime change happens the German citizens won't go quietly. Many hate the regime but many are also brainwashed into loving it. Worst I can see is Werewolf resistance to an occupied Germany.
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The brainwashing dies quickly. China will be supplying inexpensive radios that can be tuned as needed based upon demographic groups at the least (there will be differences between agricultural and industrial workers alone. North Korean youth will get access to pop culture of South Korea, which will destroy any cultural loyalty. There will be old American, British, and Japanese TV series dubbed into Korean. Don't fool yourself: China will have no use for the rabid nationalism of the North Korean regime that can be turned on a dime from anti-Americanism to anti-Chinese hatred. China is no model of democracy, but even it has a difference from North Korea: what isn't specifically prohibited will be tolerated, in contrast to the practice in North Korea in which practically everything offered is an object of mandatory belief.
The Korean Workers' Party will be outlawed as were the Nazi Party in Germany, the Imperial Rule Assistance Association in Japan, and the Fascist Party in Italy in 1945,; the Communist Party of Romania in 1989; and the Ba'ath Party of Iraq in 2003. Its political outlets will be throttled. People culpable in the worst deeds will be prosecuted and possibly executed.
And what would cause the Chinese to overthrow North Korea? Firing missiles through Chinese airspace even if the ultimate destination is the USA. So if the missile is directed at Cleveland and follows a Great Circle Route, then the missile launch is an act of aggression against China, Russia, and Canada as well as the USA.