Kind of sad they didn't just go with the original cast. I know Toph's voice actor still acts, but I guess it's possible the rest don't. I get why you can't do it if they are still children (Aang's voice actor obviously grew up and can't do child Aang's voice anymore), but they should be fine to perform the adult versions of the characters.
Iirc it was Avatar News (a leaker on X) who announced that Dante Basco would return to voice Zuko in the animated movies. Various news sites have different wordings in their CinemaCon coverage on whether Basco is expected to return, will return or wasnt mentioned at CinemaCon at all.