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.
CONPLAN 5029 is the US-ROK Combined Forces Command to prepare for the collapse of North Korea. The plan is reported to feature preparations by the South Korean and US forces to manage an inflow of North Korean refugees and other unusual situations if the North Korean regime collapses.
... In January 2005 the ROK National Security Council rejected an American proposal transform Concept Plan CONPLAN 5029 into an Operational Plan, OPLAN 5029 ... It would have given the United States command over South Korean military assets in the event of rioting, mass defections or a government collapse in the impoverished North. US officials reportedly had argued that the contingency plan was necessary to secure sensitive nuclear and military facilities, and for overall public safety.
In April 2005 South Korean Defense Authorities rejected a contingency plan that would give command authority to the United States military in the event of a North Korean collapse. South Korea's National Security Council on 15 April 2005 said it had vetoed a joint military plan with the United States on how to handle serious turmoil in North Korea, should it arise. South Korean officials said they were dropping the plan because it could limit "South Korea's exercise of its sovereignty." ... Washington distrusts the North and wants it to give up its nuclear weapons, or risk further isolation. Seoul, however, concerned about a possible collapse of its neighbor, had a policy of engaging with Pyongyang, in the hope of encouraging peaceful reforms in the Stalinist state. - OPLAN 5029 - Collapse of North Korea
Other references: Wikipedia - OPLAN 5029