I don't think *Bewitched* was the first TV show to replace one of its stars, but could we say it was one of the most notable? We went from one Dick to another Dick: it was Dick York seasons 1 through 5 and Dick Sargent seasons 6 through 8. I see York as the Diva and Sargent as the Mensch because the early Darrin is more hot headed and goofy, the later Darrin is more low key and cooperative. Maybe this is the natural progression of Darrin and Samantha's marriage?
Diva: Dick York
Mensch: Dick Sargent
Here's York in the very first episode (I don't like that it's been colorized but ah well):
And Sargent in a cute scene with Sam's parents (being a mensch and surrendering the spotlight to others):