If they really wanted regime change in NK, or a unified Korea, it's going to take the US, China, Russia, SK and Japan agreeing to it. An agreement to demilitarize a unified north of the present border, relinquish all nuke assets and research, make trade agreements with China and Russia and reduce the US military presence to a token would be a list to get them on board.
Of course China would end up having to do a lot of the work, their border isn't mined (as far as I know).
The younger South Koreans will face a generation of the burden of rebuilding northern Korea and competing with all these new people who do not share a lot of their recent history. I can predict some hostility erupted time to time, integration will take time.