Kind of like a TV version of the "(Patrick) Ewing Theory (https://www.youtube.com/watch?v=5IBa10p357E)". For example, A Different World, when it was retooled from essentially a Cosby Show spin-off starring Lisa Bonet's Denise Huxtable to a show (when Debbie Allen became the producer) with Jasmine Guy and Kedeem Hardison as the de facto leads.

That's hard and there's probably quite a bit...the first one that popped in my head was "NYPD Blue" after David Caruso left the show following his much publicized contractual issues.

There's probably a few but not most of them.

Ed.

Law and Order. The original cast was pretty dull, it improved when Jerry Orbach was made star of the law part.

Cagney and Lacey--Sharon Gless playing Cagney probably worked better than the original, with Meg Foster playing Cagney.

Although I never really watched the show, some people say that The Hogan Family (previously known as Valerie) was better after the Valerie character was killed off.

Though the main characters were STILL there...when The Facts of Life was deducted from seven to four girls and it focused more on Charlotte Rae as the adult...FOL got better breathing room.

Babylon 5.

I thought The Office actually improved for about a half season after (SC (Michael Scott) left. The supporting cast got more to do and it was interesting to see Dunder-Mifflin without Michael. However, season 9 was a dumpster fire.