the original owner of the body (heeso) is somehow responsible for what happened to dongsik (the new owner now).
dongsik, after knowing everything like the reason behind all heeso's action - made sure that heeso got the revenge against his family.
dongsik actually asked heeso if he wanted to switch body now, but heeso refused, saying he’s living his life well with dongsik’s body

So what happen to the og body? Ugh i hate this body switching trope when NONE OF THE OG ACTUALLY HAPPY. Dropped this.