The U.S. has threatened to move the two headquarters to Osan-Pyongtaek if sufficient land, about one thirds of the current size, is not allotted to them.
Thankfully, or maybe not, I've only read one person over the last few months who said why the US has to keep some troops in Seoul ---- because without a few thousand Americans in the crowded capital, American blood, so the theory goes, would not die in the initial bombardment of Seoul by the North Koreans, and thus the Americans might take the opportunity to pull out of Korea altogether, especially after they started the war with a limited strike on the North.
I'm sorry I don't have the link to the commentary by a former Korean ministry official who laid out this idea said to be flowing around the Korean government.
But, after reading the paragraphy above, I thought, "Aren't their plans to move the central government administration buildings out of Seoul already???"