The Powerpuff Girls / The Other Darrin

Pilot

The original What-A-Cartoon era shorts had Ernie Anderson, best known as either Ghoulardi or the longtime announcer for ABC as well as the voice of The Love Boat and the syndicated run of Break the Bank (1976), as the narrator, but he died of cancer in 1997. Tom Kenny took over the role.
Bubbles and then-unnamed Ms. Keane were both voiced by Kath Soucie in the What-A-Cartoon shorts, but when the series went on air, Tara Strong and Jennifer Hale took over the roles, respectively. For Bubbles, Soucie employed her usual “young female child” voice that she used for Lil DeVille and Megan and others.
The original Whoopass Stew! short, due to its status as a student film, had a voice cast made up of series creator, Craig McCracken's college friends, including a then unknown different actress for all three of the girls by name of Jennifer Freid (they spoke in unison all but once). The Amoeba Boys were also voiced by animator Lou Romano, who would later voice the father bird in "A Very Special Blossom" in the actual series.
The three incompleted shorts of that series (featured in storyboard form on the DVD set with the original voice recordings) also had different voices. Rob Renzetti voiced Mojo Jojo, and HIM (then called simply "The Devil")'s voice was provided by Zeke Kamm. In addition, Craig McCracken himself narrated all four shorts. Genndy Tartakovsky also voiced a minor character.

The show had a couple of cases of this in the transition between the two original pilot shorts and the first TV series. Tara Strong replaced Kath Soucie as Bubbles, Jennifer Hale replaced Kath Soucie as Ms. Keane, Tom Kenny replaced the late Ernie Anderson as the narrator, and Kenny also replaced Jim Cummings as the Mayor. Helped that the Mayor was a completely different character in his appearance in the short.
Carlos Alazraqui provided Lil' Arturo's voice in his first appearance. In all other episodes, Tom Kenny plays Arturo.

The series had some controversy when it was revealed that the original three voice actresses (Cathy Cavidini, Tara Strong, and Elizabeth Daily) were not invited back to reprise their roles for the 2016 reboot in favor of unknown teen actresses (Amanda Leighton, Kristen Li, and Natalie Palamides) for "creative" reasons. The actresses, as well as series creator Craig McCracken, have criticized the decision, though they do all wish the new girls well. Averted with several other characters, such as Mojo Jojo, The Mayor, Miss Keane, and the Narrator, for whom their original actors are reprising their roles. Interestingly, while Jennifer Hale does reprise her role as Miss Keane, she does not do so for Princess Morebucks (who is instead voiced by Haley Mancini).

Narrowly averted for the 2002 movie, where the three actresses threatened to go on strike if they weren't paid more, and Warner Bros. and Cartoon Network threatened to replace them, not just for the film, but for future episodes of the series as well. Fortunately, a deal was worked out, and the actresses continued their roles for the film.

Hetty Heyting and Ingeborg Wieten, the original voices of Blossom and Buttercup in Dutch, got blackballed by Cartoon Network in 2008 and 2011. They were replaced with Priscilla Kneteman and Anneke Beukman.
The Norwegian dub of the 2016 series recast the girls with new voice actresses. However even they would be replaced, with Blossom changing voice actress by season 2, while the other two getting new voice actresses by season 3. So now all three girls have three individual Norwegian voice actresses each.