Uh, South Korea doesn't even vaguely resemble its 1950 counterpart.
South Korea has a roughly 640,000 man strong army and 2.9 million in reserves. China in no way has localized military superiority given the actual size of its army at this point, and would have trouble bring up all its reserves very quickly especially given assured prompt US intervention including air power. (With the US bringing in substantial troops in the long run with possible significant international support given plainly unprovoked Chinese aggression in this scenario.)
In a scenario where China is invading a unified Korea, South Korea would also have allot of trained North Korean manpower willing to fight against a plainly non-Korean invader.