The Flash changes some events of zack snyder's films but doesn't fully retcon them. The only heavily changed film is Batman V Superman
The new Justice league going forward will be
Batman (Michael Keaton), Superman, Wonder Woman, Aquaman, The Flash and Martian Manhunter being already established and new members joining as the franchise goes on, Shazam, Batgirl, Supergirl, Mary Marvel, Black Canary, Blue Beetle, Nightwing and John Stewart
Batman (Ben Affleck), Cyborg (Ray Fisher) and The Joker (Jared Leto) will be completely erased in the new timeline
Brainiac, The Crime Syndicate and Lex Luthor will be the overarching villains of the dceu and the main goal is to bring the dc comics convergence event into live action
Henry Cavil's Superman will remain in the dceu however his appearance in the man of steel Sequel is uncertain as his role with either be supporting or his presence will be felt without Cavil's appearance
WW3, Zatanna, Blackhawks, Nightwing and Hourman will remain Theatrical while
Black Canary, Green Lantern Corps, Static Shock, Firestorm, The Justic League Dark, Deadshot and Starfire will get hbo max specific projects either shows or films
To not overlap with the reevesverse. Heroes and villains of The Batman won't be used in both universes (no catwoman and riddler in the dceu, not Batgirl and nightwing in reevesverse