Depends on if it is at the federal or state level.
At the federal level, one rather historical location is Elliot county KY. It voted for the dem candidate every election from 1872-2012, with Obama winning by less than 3 percent before giving 70% to trump. It also backed the dem in every congress, senate and governor election since the 1960s. In 2014 they voted for Kenneth Stepp over incumbent republican Hal Rogers. In 2018, they went for Hal Rogers. These voters were of course historically moderate democrats who loved trump's populist appeal.
Another notable example could be a democrat from Michigan who supported Gary Peters for the US senate in 2014 won by 13% but who then supported John James, who lost by just 6%.
And finally, someone from New Jersey who found Bob Menendez too corrupt to reelect.
The state level is another story.
Here the most likely case was an open governor's election where a voter supported a dem over a republican who narrowly won. They then thought the incumbent republican was doing a good job and thus supported him in 2018. This for sure happened in Vermont, Massachusetts and Maryland.
In Florida, Desantis flipped two Crist counties. These voters were probably willing to vote for a republican turned democrat with Charlie Crist but found Andrew Gillum to progressive.
Also could someone explain how Monroe county Ohio went from Ed Fitgerald to Mike Dewine?
The other county I can't at all understand is Jefferson county Kansas, which supported Paul Davis over Sam Brownback but went back to the GOP with Kris Kobach. If they hated the Brownback so much, why would they support Brownback X10?
Monroe county ohio also has a plant close down and like 25% unemployment so they went with the #populist route
