It’s a comic trope that goes back decades, from episodes of “Gilligan’s Island” and “Star Trek” to the Rob Schneider movie “The Hot Chick”: A man and a woman trade bodies. These fish-out-of-gender stories usually derive much of their humor from stereotypes about men and women, and end up reinforcing our received notions about the gender binary. So it’s especially welcome that “People Collide,” a fascinating new novel by Isle McElroy, the author of the acclaimed “The Atmospherians,” brings new life to the conceit, turning it into an opportunity to interrogate not gender but class.