Out-Of-Body Experiences

Although Cha Min does leave his body for a moment in the beginning of the drama, I'm more addressing the whole "new body to match your soul" thing our main character have happen to them due to a mysterious orb from mysterious origins. ... and that basically sums up this reasoning box...

Cha Yu Ri has passed away after childbirth. Wanting to see her child grow up, and watch over her husband who is taking the loss hard, she sticks around as a ghost.
However curing the deity for her fate one day, she is struck with the punishment of no longer having the usual properties of a ghost. Basically, she can interact with everything she could when she was alive.

Wen Shuai, here in after called Daniel because I read the webtoon first and my brain refuses to move on, is "ugly" and "fat" and because of this is extremely bullied. After a huge incident he moves to a new town and changes schools. However just before he goes to school he suddenly wakes up to find himself in a perfect body. After some experimenting he finds every time he sleeps, his body switches back and forth between bodies.

Shin Sun Ae is a virgin ghost. She wants nothing else but to spend a night with a man before she'll let herself pass on. In order to do so, however, she needs to possess a woman's body. Thus appears Na Bong Sun. She's always been able to see ghosts, but this is the first time she's been possessed. Anyway, Sun Ae occasionally pops in and out of her body.

Kwon Shi Ah and Shi Yeon are twin sisters, but complete opposites who fight and can never see eye to eye. Thinking less of each other, the two suddenly find themselves in each other's bodies. Thinking they either have it easy or can live the other's life better, the two find that their sister's life has just as many hardships, although differently shaped. 

Woo Jin has an interesting condition. After every night, when he sleeps, his body changes to a completely new face, gender, age, ethnicity: he becomes a completely different person. He still knows Korean, but can suddenly find himself speaking a whole 'nother language.

Da Xi and the emperor, Ji Man, after a misunderstanding and an accident, find their bodies and souls are switched. 

Go Ha Jin, in an attempt to save a child from drowning, almost drowns herself, and suddenly finds herself in the Goryeo Dynasty in the body of Hae Soo. Same actress though! 

Head chef for the Blue House, Jang Bong Hwan finds himself in trouble, and soon wakes up in the Joseon Dynasty, in the body of soon to be queen, Kim So Yong. Understandably troubled, he needs to get used the time period, political intrigue, and being in a woman's body.