User blog comment:Liquidbabydoll/Retiring characters/@comment-961279-20141217150934

Some of it is symbolic, like when a sports team retires a jersey number when a memorable player had that number. Another part is making an evaluation of do they still need that particular character for one or more stories versus can they use a different character instead or can they tell the stories in a different way without that character. If the answer is they still need that character, then a new voice actor is found.

In the case of Zapp Brannigan, Phil Hartman had been killed before that first episode with Zapp aired, so the show had time to find a new actor before a particular style of voice could have become locked into the public's mind. Same kind of thing with Chris Farley and Shrek.

Compare that with Dukey from Johnny Test. Dukey is a major character in the show, which means that when the original voice actor left, they had to find a new one. While the second actor has made Dukey's voice close to what the first actor had, you can hear the difference and it stands out.