Yeah, but you run into bad-blood and potential bad-press to be buzzed around Tinseltown when you don't sever ties completely with a disgruntled actor. With Richard Harris, he died before his character could help Harry's character arc be completed to the books. He was a crucial element to Hogwarts. With DeMunn, AMC was afraid of anymore bad publicity, people in the Darabont camp might gripe about.
"I didn't like it, So, they replaced me."
AMC publically let him do a honorable thing for his friend (surprising spin) and probably figured they could just write in a new character anyway, like Merle, Daryl, T-Dog to expand the television universe past the funnybook.