I can say it wasn't working for me with 18.1 final... but I tried it on iOS 18.2 beta and I was able to switch my primary address to the main iCloud address I was using. I waited a few days... and just deleted the previous iCloud.com primary address from my account. And nothing broken
that said, my Apple TV is showing the "new" address in settings.
but my Mac and iPhone are still showing the old address in different places... and everything is still working fine. I guess if I disconnect and reconnect the account, it will update to the new address.