They probably keep it secret, but what is known about South Korea's plans if the North Korea regime suddenly collapses? Or what have observers/scholars suggested will happen?
They indeed have a plan, known as CONPLAN 5029. It is drawn up in coordination with the United States, and takes into account multiple scenarios. A few years ago this "concept plan" was proposed to be transformed into an "operational plan", OPLAN 5029, but the South Korean Security Council rejected this. They have since signed an agreement that mimics the intentions for contingencies accounted for in CONPLAN 5029.