Devil Doll
(1964)

Pervy stage ventriloquist Dr. Vorelli, secretly murdered his assistant Hugo and forcibly transmigrated his soul into a butt-ugly puppet, garnering Vorelli worldwide fame and riches.

The understandably pissed-off Hugo, however, is forced to perform all sorts of sordid and demeaning acts for Vorelli.

A climactic battle of wills results in the classic switcheroo: Hugo now occupies Vorelli's body while Vorelli is trapped within the dummy.

Thanks Moore!