...of established characters?

If one or several main characters left, for instance.

Three's Company did just fine for many years after Suzanne Sommers. Alice also prospered for many years after Flo (Polly Holliday) left.

The Facts of Life started with around ten girls. They got rid of most of them early on to concentrate more on individual characters. Molly Ringwald was among those cut out of the show.

M*A*S*H

Most of the 80s and 90s nighttime soaps went on for years with major cast changes; I'd argue that Knots Landing got a little better, but that Dallas and Dynasty got worse, quality-wise.

Newhart lost Steven Kampmann and Jennifer Holmes and gained Peter Scolari and Julia Duffy. Although the scripts kept getting screwier over the years, those were good changes, particularly losing the Kirk character.

Night Court