There are those who believed that the Eigth Army had its troops set too far forward to ease South Korean concerns that we would spot the DPRK half of the South's "sacred soil" to save our forces for later.
In other words, there were no valid miltary reasons to keep the troops that far up, in the teeth of the DPRK's massive artillery batteries.
Moving the bulk of the Second Infantry Division and it's auxiliary units south of the Han provides that unit (and, hopefully, most of the South Korean Army) with a natural defensive line. Most of the northern batteries are in fixed positions, so in the first days of a North Korean assault, the North Korean Army will be shielded by a massive artillery blanket.
However, unless the North Koreans can easily move their batteries with their mechanized infantry, the level of artillery protection will necessarily decrease as the DPRK troops move south to meet the Eighth Army MLR.
The idea is to maintain the unit cohesion of Eighth Army and slow the momentum of the initial assault, which will involve well over a million North Korean soldiers. We need about a week for two things to happen: deployment of Third Marine Division from Okinawa and redeployment of CONUS based air assets to the Pacific Theater of Operations. Time is everything, here. If we can apply American airpower and stall out North Korean armor, we will win the war. Decisively.
All that will be left to do is the assembly of an American field army in Japan for an Inchon-type invasion in the North Korean rear.
Be Seeing You,
Chris