In the English dub, a voice actors' strike happening at the time meant that none of the returning characters from the original Mobile Suit Gundam had their original voices.
Many of the female characters had their Japanese voice cast replaced for the Compilation Films.
Reality Subtext: According to backstage rumor, Reccoa and Scirocco were at least partly based on voice actress Maria Kawamura and character and mecha designer extraordinaire Mamoru Nagano, who were the other two thirds of a Love Triangle Tomino was involved in during production. They got married, actually, and still are.
Zeta seems to be a veritable lightning rod for backstage drama. There's also the infamous recasting of Four in the compilation movies, allegedly due to a Casting Couch arrangement between Yukana and the director, as well as Rosamia's much reduced role as a result of the casting director having been married to her original voice actress Kyouko Fujii and then divorcing messily, resulting in her being replaced by Yuu Asakawa.
Shrug of God: During the Gundam 30th Anniversary Expo of 2009, Nobuo Tobita revealed how Director Yoshiyuki "Kill 'em All" Tomino warned them since Zeta Gundam is an original, no matter whether they're The Protagonist or not, nor how cool their characters was designed, if he felt any of the voice actors showed "signs of weakness in performance" during production, he would gleefullykill them offanytime he wants. Which unsurprisingly scared the bones out of Tobita himself, and mentioned he really felt like a true Action Survivor once the whole production was over with his role making it out alive in one piece.
Originally, Gates was supposed to die together with Rosamia. And then he was going to survive and reappear in Mobile Suit Gundam ZZ (which would have presumably involved a great deal of Talking To Yourself). He never appears in ZZ Gundam but his final fate is left ambiguous.
Mamoru Nagano was originally going to have a much larger role in the creation of the series, but due to a falling out with Tomino only a few of his mecha designs, most notably the Hyaku Shiki, ended up getting used. Nagano's version of Zeta would have included:
Axis Zeon having a much wider range of mobile suits besides the Gaza-C and Qubeley, including a gigantic mobile fortress known as the Nahatgall, created as a counter to the Titans' Psycho Gundam, that later became the basis of the Mobile Suit Gundam: Char's Counterattack novels' Nightingale, as well as the Rick Dias and Galbaldy Beta being Zeon MS rather than AEUG and Federation, respectively.
Haman Karn having a completely different character design with white hair and a flamboyant red and white costume similar to FSS's Mirage Knights.
The Hambrabi (which was originally known as the Methuss) being an AEUG MS designed as an artillery spotter for the Zeta's Hyper Mega Launcher. This gets a nod in Gundam Vs. Zeta when Four pilots it alongside Camille in an alternate timeline where Camille joins the Titans.
In an abandoned draft of Char's Counterattack, Jerid Messa was meant to survive and become Amuro's wingman...then ZZ Gundam came in the way.
Speaking of Jerid, there were plans at one point during Zeta's production to have him turned into a cyborg by Haman.
Emma Sheen was supposedly slated to return in ZZ Gundam as well.