I expect Aquaman (outside Heard), Blue Beetle, Pacemaker and Shazam to not have any cast changes as long as the movies planned for next year don't flop. Peter Safran who is now co-chairperson and CEO of DC Studios was the producer of them pre-getting this new job, so can't see a way these movies/shows don't mostly line up with the new plan.
As for other casts members, my guesses are.
The whole Superman cast is gone. I could maybe see Laurence Fishburne, Kevin Costner and Diane Lane coming back if they plan to use Flashpoint as the story reason for reboots.
Gal Gadot, I'm 50/50 on here going. She isn't good in the role however she is going to be in Shazam 2 and has had a big push in Hollywood. If she goes it will be due to her upcoming $20million pay day for Wonder Woman 3 or her age rather than anything else.
Cyborg is gone. Less to do with the actor and more he isn't most interesting character. Already been hinted at something with Mister Terrific in the works, who could easily take other the tec role Cyborg has in the Justice League.
Erza Miller is gone, rest of the Flash cast no idea about. Main question is will the Flash still be Barry Allen or in a recast will they change it to be Wally West.
Suicide Squad cast for both movies I doubt they will change, unless they plan to use the Joker again then I do see a recast. James Gunn could have recast actors in his own SS movie but decided against it, going so far to change Deadshot to Bloodsportto avoid this.
I see them keeping Ben Affleck as Batman but rather than give him his own movie he just has smaller roles in other movies or in the Justice League. It seem they have no control over the new Batman movies and I doubt Matt Reeves will want to merge his movies into this new DCU, so all they can really do is use Ben Affleck or 71 year old Michael Keaton without stepping on the toes of Matt Reeves.
Black Adam is likely gone, could see they maybe throwing another bone to Dwayne Johnson for him to be in Shazam 3 or movie title Shazam vs Black Adam but at a reduced fee. But from what I hear him rejecting a cameo in Shazam 2 may have put an end to that. As for the JS members, I just don't expect to see them again.