I know that the Dick York to Dick Sargent switch (https://tvtropes.org/pmwiki/pmwiki.php/Main/TheOtherDarrin) occurred because York's health stemming from an old back injury could no longer allow him to properly work. Why couldn't they have just written the character (https://web.archive.org/web/20061031125231/http://www.jumptheshark.com/b/bewitched.htm) out? I'm assuming it was because:
The writers did not want to have a storyline involving Samantha being widowed or divorced (had the show lingered later into the '70s and Dick York left the series later) and then re-married.

In physical appearance, Dick Sargent didn't look that much different from Dick York.

https://static.tvtropes.org/pmwiki/pub/images/bewitched_darrin_7688.jpg

Bewitched was first and foremost a love story & to have half of that love taken away would have made that premise moot & a non sequtar.

It would have made it a completely different show with an altered premise, Bewitched in effect would have no longer existed.

In addition for the premise to be so effective the love between Darrin & Samantha would have had to have been so special and unique that neither could have found nor accepted an alternative to the other in love or life.

For Bewitched to continue the part of Darrin had to be recast and thank God it was as we were entertained with three more superb seasons of a great show, Dick Sargent was excellent in the role & did a great job & made it fresh and new again.