This is definitely the one thing that U.S. and South Korean military planners worry about. The NK navy and air force will cease to exist within 24 hours of hostilities, then the air defenses will be worn down over the next several days. Their ground forces, while large, have no real operational experience in today's high tempo operations and without air cover are going to suffer horrific losses. The issue is with the artillery. I read a few years ago in a study that with ROK, U.S., and Japanese aircraft flying strikes 24/7 that it would take at least a week to knock out all of the artillery in range of Seoul. By that time the city would be mostly rubble.
I have little doubt that SK will mop the floor with NK but will China come to their rescue? Border tensions with China and NK have been high but that does not mean they will not aid them.
China has their own problems now though... wars often come on the heels of financial problems.
History (1940 blitz of UK, WW2 bombings of Germany and Japan, burning of Moscow in 1812) shows it takes more than trashing a big city to win a war. A destroyed Seoul would be an economic and humanitarian catastrophe, but wouldn’t change the overall outcome of the conflict. Anyhow, here’s a pretty good article that questions the hype over just what the NORK’s artillery could really do to Seoul. http://www.popularmechanics.com/military/a6212/north-korea-and-flattening-seoul/