Mo Rocca's podcast, Mobituaries, has a January episode titled Sitcom Deaths and Disappearances. The middle section deals with the Darrin switch. Mo's special guest is Lila Garrett, who wrote many scripts for Bewitched. Her take on the switch is fascinating (to me, anyway), and very satisfying indeed, especially if Dick York is your favorite Darrin. (Dick Sargent recieves praise, too.) The section begins about 17 minutes in and ends at about 30 minutes.
The whole podcast is worth listening to. The first section deals with characters who disappeared, never to be mentioned again. The final section is about sitcom characters who died (although the actor lived).