The extra four kids with Angemon are voiced by Joshua "Tai" Seth as Teppei, Brad "Kazu" MacDonald as Katsuharu, Tifanie "Yolei" Christun as Chiaki, and Brian "Davis" Donovan as Teruo. Dave Mallow reprises his voice role from Digimon Adventure as (a different) Angemon.
Mary Elizabeth McGlynn, was given the opportunity to play a larger role than just a digivice shouting "Digivolution" (or "Matrix Digivolution", like she also did in Tamers), leading her to voice Ophanimon.
There's an interesting case related to Cherubimon's voice actors. Paul St. Peter voiced Diaboromon in Digimon: The Movie, and by extension, the Diaboromon-infected (at least in America) Kokomon, and Cherubimon (Kokomon's highest form), and he is later casted as the first FrontierBig Bad, Cherubimon. When he returns to Lopmon, St. Peter is taken over by Michelle Ruff, who was Tamers' Lopmon, in a similar "former agent of darkness reverted to friendly" setting.
Channel Hop: Because of the fact that Disney bought out Fox Kids/Saban a year prior, this one ended up airing on ABC Family and in the last season of Disney's weekday/sunday block on UPN stations, Disney's One Too (a spinoff of One Saturday Morning over on ABC), though by this time it had dropped that name.) It likely didn't get onto ABC because Disney was contractually obligated to dub this season, and had no interest in the show otherwise.
Frontier didn't get aired in either Malaysia or Morocco. In Malaysia's case, it's kinda odd that this is the only series of the franchise that wasn't released, while the others didn't have any problems to do so.
The Other Darrin: Due to fear of damaging his voice from all of the shouting, Michael Reisz, Takuya's English voice actor, refused to voice EmperorGreymon, giving the job to Dave Wittenberg instead... only for there to be not nearly as much shouting for EmperorGreymon's scenes. Wittenberg also replaced Reisz as Takuya for the movie.
A similar situation became close to happening for the Japanese version, when, exhausted from doing all of the pattern voices, Junko Takeuchi (Takuya) asked Hiroshi Kamiya (Kouji) to voice Susanoomon for her. However, Kamiya refused because Takuya is the main character and she should've do it, and the director overruled the request; as a result, Takeuchi voiced Susanoomon as it was originally planned.
The first character design sketches released in the V-Jump magazine showed some of the characters with different color schemes. Additionally, Kouji was originally going to sport long, black slacks instead of the grey capri sweatpants he eventually dons in the show. One of the fanbooks reports that Kouji was originally going to be a female character, an idea that was presumably discarded because the previous season also had a female lone wolf (Ruki Makino).
Crispin Freeman was originally considered for Koji in the English dub, but wound up playing Koichi instead.