La Beauté du diable (1950)

Henri Faust, retiring after 50 years as a professor in a circa-1700 French university, despairs at the ravages of old age ... whereupon Mephistopheles, agent of Lucifer, appears as a virile, handsome young man and exchanges bodies with him to induce Faust to sign a pact to exchange his soul for renewed youth, riches and power. But though the "new" Faust is attracted by the material improvements in his life, he remains wary of signing, while Mephistopheles, now posing as the aged professor whose body he inhabits, must find a way to trick him into signing the pact - and dissuade him from the love of a gypsy girl who prays for his soul - or find himself damned by his own Master...