The Bewitched casting department did an incredible job replacing Darren. Totally seamless for the viewer.
In old timey radio, the casting department for the program, The Great Gildersleeve managed an equally seamless replacement when they found Willard Waterman to replace Harold Peary in the middle of a run that lasted half a generation. Not only did they find an almost identical voice actor, the resemblance between the two was remarkable.
Quick find and a perfect hint.