You have specified an additional root Azure AD domain to be federated. To configure federation successfully, we have to update ...

You have specified an additional root Azure AD domain to be federated.  To configure federation successfully, we have to update the existing federated domain with the settings for multiple domain support, as well as delete and re-create the relying party trust in AD FS.  During this process, users will be unable to login. Any subdomains already configured will require additional updates after configuration.