When Korean war happened, China was helping NRPC out. Now they still have a tight relation with each other. So by signing defense agreement with China, South Koreans would be feeling safer now?
It is very unlikely that the Kim regime will want to change anything to the level of South Korea the best you could hope for is that they rise to the level of China. The reunification of the Koreas would mean the end of their regime because the South Koreans are far too liberal and would spread that in their country. They will do everything in their power to make sure that doesn't happen while enriching themselves.