Except for the bad guy. Excellent example is Deadwood - where all the characters change except for Hearst, who is a true sociopath (or more helpfully, a kind of sorcerer who works for some subterraneal entity in search of THE COLOR, that is, GOLD.)

If your good guy is bad - eg Al Swearengen - make the bad guy really bad. eg Hearst and Cy Tolliver. In the show, we see all the characters grow, and we’ve identified with them. Good storytelling.

Tolliver changes, but for the worse. One of the saddest, most pathetic creatures I’ve ever seen in cinema.