Yep, someone has access to the demo build, though they don't appear to be interested in releasing their dump online (and the demo itself is mostly missing content past World 1 anyway, so no fear of endgame spoilers for now). I've gone through the provided cast list and have most of the roles down:
By process of elimination here our new Mario and Luigi is, most likely, Mick Wingert! Given there's only three male American actors and one is Kenny James, technically it's worth considering Afghani as well; but his Genshin character sounds pretty close to the flower so i'm fairly confident.
As for the rest; Samantha Kelly, Kenny James and Caety Sagoian are here; the latter two are certainly reprising their usual roles. Technically it's possible, given she has multiple roles, that Samantha Kelly is only voicing the Toads now and Peach is someone else, but I think it's fair to assume it's still her for now. Yoshi and Daisy appear to have been definitively recast though; disappointing Deanna Mustard hasn't been given any fanfare at all if she'll no longer be involved with the series, her and Kazumi Totaka were the longest running Mario VAs behind Charles himself. Nabbit was already given a different voice awhile ago with NSMBU Deluxe and Mario Kart Tour but neither game has voice credits so we still don't know who it is (or if it's even the same person now).
All of the remaining American VAs have voiced for Fire Emblem before, so they'd certainly be on NoA's list. Pick between Daisy, Yoshi, Nabbit, Prince Florian and the Poplins (possibly Peach if you still think she's been recast) for who they could possibly be voicing.
If Rosalina is somewhere in the game like people expect she'd need to be recast; Laura Faye Smith isn't here either.
Mick Wingert voices Po from Kung Fu Panda in TV series and videogames. Po in the movies is voiced by none other than Jack Black Bowser. Bowser and Mario is the same person CONFIRMED!