Seconds (1966)

Seconds directed by John Frankenheimer and starring Rock Hudson. What happens when you don't know what you want, and then you get it? I had never heard of it until I came across it on a list of 100 Best Science Fiction films, which describes it as "one of the most radical, disturbing and downright terrifying thrillers ever released by a major Hollywood studio". I agree it's disturbing in its way, though not as much now as it must've been at the time. It's considered a cult film. I watched it at Internet Archive:

Deep Focus Review concludes, "Seconds holds a deeply human message and marvelous cautionary tale, ever hopeful to viewers who heed its warning." Criterion opens with this: "Rock Hudson is a revelation in this sinister, science-fiction-inflected dispatch from the fractured 1960s." Slate says, "Oh my God, Seconds" and "Seconds is one of those movies that has somehow held on to permanent cult status."

Moria says, "Seconds is an astonishingly bleak film. The deliberation of John Frankenheimer’s storytelling and the dispassionate arm’s length he places himself at leads to something cold and chilling." 366 Weird Movies says, "Dramatic and disturbing, Seconds is a dark, brooding predecessor to middle-class America mid-life crisis films..."

Roger Ebert says it "will linger a lot longer than the title suggests in the mind of anyone who chooses to watch it. In fact, it might be one of the most haunting American films to come out of the 1960s, or any decade for that matter." Rotten Tomatoes has an audience consensus score of 87%.