Walk in's are always prearranged. One spirit walks our the other takes over the body, and they are rare.
Let's say one spirit needs to experience something in the early part of a life time, and an other doesn't want go through the birth process and the long growing up process.
In my view, in the ways I understand the soul, a soul exchange does not exist, since a soul never moves from where it is. And it does not matter where in the universe the spirit would move, a soul will always be in the same position. The only way it will move it up in vibration.