Law & Order lasted a long time, 20 seasons. Almost every season there was a cast change.

Season 2 Paul Sorvino replaced George Dzundza
Season 3 episode 9 Jerry Orbach replaced Paul Sorvino
Season 4 S. Epatha Merkerson replaced Dann Florek and Jill Hennessy replaced Richard Brooks
Season 5 Sam Waterston replaced Michael Moriarty
Season 6 Benjamin Bratt replaced Chris Noth
Season 7 Carey Lowell replaced Jill Hennessy
Season 8 the first season with no cast changes
Season 9 Angie Harmon replaced Carey Lowell
Season 10 Jesse L. Martin replaced Benjamin Bratt
Season 11 Dianne Wiest replaced Steven Hill (the last original cast member)
Season 12 Elisabeth Rohm replaced Angie Harmon
Season 13 Fred Dalton Thompson replaced Dianne Wiest
Season 14 just the 2nd season with no cast changes
Season 15 Dennis Farina replaced Jerry Orbach
Season 15 midseason Annie Parisse replaced Elisabeth Rohm
Season 16 no cast changes
Season 17 Milena Govich replaced Dennis Farina and Alana de la Garza replaced Annie Parisse
Season 18 Linus Roache replaced Fred Dalton Thompson and Jeremy Sisto replaced Milena Govich
Season 18 episode 15 Anthony Anderson replaced Jesse L. Martin
Season 19 and 20 no cast changes
In the early seasons Carolyn McCormack was in the opening credits. When she left she wasn't replaced.

I'm glad Milena Govich replaced Dennis Farina. I couldn't stand him.

Weird, I liked Dennis Farina and didn't like that Milena Govich, I'm glad she wasn't on the series long.

Ed.

the show was like the Saturday night live of drama shows with all of the cast changes

Was that always the intent of the show, to change cast members every year to make it fresh for the viewer instead of going stale?