Major Party Nominees who voted for Opposite party in later election

[Alben Barkley]

Wouldn’t surprise me if Eisenhower secretly voted for LBJ in 64 or LBJ himself voted for Nixon in 72.

I agree on LBJ for Nixon, but I think Eisenhower was truthful about suppporting Goldwater since he really liked the also very conservative Reagan, advising him and supporting him behind the scenes. (https://www.washingtonexaminer.com/weekly-standard/unearthing-the-eisenhower-reagan-connection)
There's also the fact that he did an ad supporting Goldwater where they appeared together (https://www.youtube.com/watch?v=Lly3jANwMZM) which I doubt he would have done if he didn't actually support him.

LBJ hated Nixon's guts and would never have voted for him.