Steve Blum was also replaced as Vilgax due to schedule conflicts, the role being given to James Remar, who himself didn't voice the character in Vilgax Attacks!, one of the video games, where he was instead played by John Dimaggio, whose rendition of Vilgax is much closer than James Remar's. Steve finally reprised the role in Omniverse.
When Vilgax and Azmuth use the Omnitrix in the episode Primus, their transformed states use the same voices, rather than the usual guy.
During their appearance in the original series, Ben's parents, Carl and Sandra, were voiced respectively voiced by series regulars Dee Bradley Baker (Stinkfly, Eye Guy, Wildmutt) and Tara Strong (Ben, Ben as Upgrade). In Alien Force, they're replaced repsectively by Don Mc Manus and Beth Littleford, who played the roles in Race Against Time.
Postscript Season: Across its first two seasons, the show tells a sprawling epic storyline of a battle against one of the oldest races in creation, culminating in a final episode in which virtually every single dangling plot thread, character and story concept the series has set up is convincingly brought together for a dramatic final battle that ended with the saving of the entire universe....aaaand then they made a third season. The season distinguished itself by bringing back the Big Bad from the original series, but ultimately achieved little other than setting up the next incarnation of the series with its finale, consisting almost entirely as it did of one-shot adventures of no consequence, with little to no use of the new ideas and concepts set up in the season premiere. The third season was quite clearly not originally planned for the final moment of season two, in which Ben looks at the Omnitrix and claims to have a new set of ten aliens he does not recognize: quite obviously intended as an open ending that was then ignored when season three began and he had his standard ten still in place.
What Could Have Been: One of the writers said that there was a big story planned for season three that involved Kevin absorbing more energy (presumably to save Gwen) and fighting against the insanity that comes with it but ultimately becoming evil. Cartoon Network approved it at first but then they chickened out at the last minute, so the writters had to scrap that story and start a new arc for the season from scratch. This same story was rescued for the first season of Ultimate Alien but with a different approach, in particular with a happy ending.
Also, Man of Action's original idea for the sequel series was a teenage Ben and Gwen would become mentors to a group of kids with mysterious alien powers. When they were replaced by Murakami and McDuffie, the concept was changed to the Ben, Gwen, and Kevin teaming up to fight the Highbreed instead, with the "Plumbers Helpers" being the main leftover from the old concept.
Word of God: One of the main writers, Dwayne McDuffie, has a thread where he answers questions and brings foreshadowing to the series. Anyone who is majorly confused with the continuity errors in this show should check this out). It addresses many oddities and seeming plot holes.
Sixsix and Sevenseven's language is too complex for translators to handle.