The recent change of attitude by North Korea towards South Korea could be attributed to a number of obvious factors like the continued collapse of the North Korean economy along with morale and effectiveness of the North Korean military and security forces in general. Recent defectors from North Korea report that conditions inside the military are bad and getting worse. Physical exams of these defectors confirms those reports.
But there is another reason for the change of attitude that was not commented on much because of the sensitive nature of the information. In short, at the end of 2017 North Korea got a look at South Korean and American war plans and were alarmed at what they found. The northerners had every reason to believe this information was accurate because in late 2017 South Korean military networks were hacked and a large quantity of secret documents appear to have been copied. This apparently included several OPLANs (Operational Plan, a plan for a single or series of connected operations to be carried out simultaneously or in succession by specified military units). American OPLANs after World War II often involved allied forces and there are a number that involve South Korea. The hackers are believed to have taken OPLAN 5015 (joint U.S.-South Korean response to all-out war with North Korea) and OPLAN 3100 (South Korean only responses to various North Korean local provocations.) OPLANs are typically updated frequently (even daily) in times of crises so what the North Koreans may have got could already be out-of-date. However OPLANs also include a lot of operational details that do not change much over time so grabbing even one version of a particular OPLAN has its uses.
South Korea and the United States have a number of shared OPLANs. Some of these are more like disaster plans but involve military units. For example OPLAN 5029 deals with making sure North Korean nuclear weapons technology will not get into the wrong hands (the U.S. will deal with this), and that preparations are in place to deal with the North Korean army falling apart, or millions of hungry North Koreans trying to move into South Korea (South Korea takes care of this). There are also plans for dealing with natural disasters that do a lot of damage to both countries. Another shared operation is OPLAN 8044 that covers nuclear retaliation for a wide variety of situations. This OPLAN has long included details of which nuclear missiles were aimed where in Iran and North Korea if the need arose to fire back.
In response to this latest hack South Korea quickly ordered some changes in its Cyber War defenses. OPLANs were apparently also updated to minimize the damage done by North Korea possibly having some current as of late 2016. What was scary about all this from the North Korean viewpoint was that the OPLANs detailed capabilities many North Korean generals believed were enemy propaganda. But OPLAN documents are top secret and only for internal use. No need for propaganda there and that made it clear the North Korean military was a lot more vulnerable than North Koreans realized. The South Korean and American intelligence knew a lot more about the location and status of North Korean weapons than the North Korean generals had believed. Not only that but the OPLANS described in detail how the many modern weapons the South Koreans had, like smart bombs and guided missiles, would be able to do a lot more damage to the North Korean military and do it faster than the North Koreans had believed possible. The OPLANS described how the North Korean air defense system would be quickly destroyed and South Korean and American commando teams would hit key targets. OPLANS made reference to messages broadcast to North Korean civilians emphasizing help (food, medical care, elimination of the police state) was on the way. While many North Koreans would fight to defend the Kim dynasty the North Korean secret police (that monitored public attitudes) knew that a growing number of North Koreans would welcome the southerners as liberators.
Once the North Korean hackers delivered the stolen OPLANs documents in September 2017 it took a few months for the military and other security agencies up north to digest all this information and conclude that the north was screwed. Supreme leader Kim Jong Un was briefed, followed by him firing another few senior advisors who were apparently on the wrong side of this new reality. Kim then told South Korea that he wanted to improve relations, send a delegation to the Winter Olympics and get together with South Korea leaders to have friendly discussions about matters of mutual interest.