ok I try to answer has best I can. but I my be wrong so...
from what I know it seep in their culture of the korean that ghost's are not only exist but also can stay or need to move on somehow. the same we can see a lot in Japanese manga.
and I think the start of the story - cause they died in the same way - been killed by a track., but wish for something the other have. then the "God" or whatever give them their wish or it was the Circumstances that make them switch places to get what they wanted - one wanted to die and leave this world and the other wanted to live with all his power.
and it not the first time I seen soul been switch with other body, I seen a lot of anime's doing the same thing and never explain how or what happen. but I hope my explaining was enough, at least that what I think happen ╮( ̄▽ ̄)╭
did it make sense???

Seriously ... I'm speechless ! I'm so fucking disappointed !!!! -_-
This is not enough at all to cover everything, even with those side-stories ! It's so incomplete, so rush ... There still to many questions !!!
Well, Fuck....