How big a worry would insurgency be?
Insurgency will definitely be a problem. Juche has always had religious aspects to it, and for many North Koreans, the Kims are effectively gods. A lot of North Koreans will be easy to deprogram, and counter-insurgency will be able to smash most of the rest, but pro-Juche terrorism will be a problem for decades to come. After North Korea is officially defeated, I think the KPA would transition to a military force comparable to the FARC or Shining Path, which like both groups, will decline in power and influence as the government gradually eats away at their numbers and support base over the years.
And without a border between the North and South, these terrorists would easily be able to strike within South Korea, so it wouldn't just be North Korea which has to deal with these guys. Basically, the Republic of Korea is going to be a lot less economically powerful than it is today, although former North Korea will be a lot better off.