It was an unexpected transition. In Back to the Future (1985), when Marty McFly (Michael J. Fox) travels back in time from 1985 to 1955 and then tries to convince Dr. Emmett Brown (Christopher Lloyd) that he really was from the future, they have this exchange:
Dr. Emmett Brown: Then tell me, "Future Boy," who's president of the United States in 1985?
Marty McFly: Ronald Reagan.
Brown: Ronald Reagan? The actor? [snorts in disbelief] Then who's vice president? Jerry Lewis?
[He rushes outside, down a hill and toward his laboratory]
Brown: I suppose Jane Wyman is first lady!
McFly: [following Brown] Whoa! Wait! Doc!
Brown: And Jack Benny, the secretary of the treasury.
McFly: [outside the lab door] Doc, you gotta listen to me.
Brown: [opens the door to the lab] I've had enough practical jokes for one evening. Good night, Future Boy!
Actually, Reagan was already married to Nancy Davis by 1955, but we'll give the good doctor (and his scriptwriters) a pass on that. It's hard to keep up with who's married to whom in Hollywood.