I personally wouldn't be surprised if this new anime is pretty much just a Anime version of the Live Action series, so all the events and changes that happened in the LA version from the original Manga/Anime happened in this new adaptation.
Otherwise I am seeing this as more of a DB Kai situation where they make it a lot more condensed.
As for the cast, I too have considered the possibility of the Live Action cast playing their characters in this new Anime (and it would make it even more funny if my theory is correct and they have this new series as an Anime version of the Live Action series)
However, the vast majority of the CR cast of OP are union qualified anyway, so they could still return (not sure if they will since they are obviously busy doing the OG series anyway) though if the sub cast is returning, it would make sense for the dub cast to return as well. Being an ONA rather than a Simulcast/Simuldub should be a lot easier to handle dubbing. Plus since the CR cast returned this year to dub the Black Clover movie even though that was union and handled by Netflix, there is no doubt in my mind that they are indeed able to get the OP CR cast back since the majority of the Black Clover cast are in One Piece.