The question has been answered?

Realistically, you can't switch your first assignment. But you can ask for a compassionate reassignment. Asking for Korea isn't too hard to support, even for a 68E. You can't switch with the prior service person because they are going TDY enroute, it's a totally different pipeline and they've already shipped their household goods before they arrived at the school. You can also try asking the branch manager, Korea is not a frequently requested position and they are always having trouble filling it.

Keep in mind that when you Fail an MOS/Leadership school you are assigned according to the need of the service