Request from... someone
Great Shift: Most of the world's population permanently shifts into random bodies.
Exchange Island: A special vacation spot in which you spend the week in another visitor's body.
Swap Class: A controversial high school class many parents force their children to take; spend some time (usually 2 weeks) in a classmate's body of the opposite gender, and become a more "open-minded" person.
Long Distance Body Swapping: To save time, some companies have potential employees from far away swap bodies with one of their own (sometimes female) employees for an interview. Swap facilities book up quickly, so an overnight stay is often required.
The Fantasy Orgasm Swapping Event (FOSE): Thousands of people having an orgasm to a fantasy suddenly swap bodies with that person due to a strange cosmic radiation that passes through the Earth (MY IDEA! Feel free to use, of course).