Even if a unified Korea allow US base to stay in a unified Korea, I doubt Korea will let US base move closer to the Chinese border, it is a provocation to do that, and has no benefit whatsoever to the Koreans or even to the US. So the US will probably stay in their current base, which is not much different that the current situation, except in a unified Korea, the US will have a hard time to justify its existence there other than to maintain its hegemonic ambition.