@_elena simply generating a new user does not forward the old user to the new one... it simply generates a new user that can be followed... I am working on a re-naming feature and a migration (Move-Activity) https://github.com/Automattic/wordpress-activitypub/pull/685
If you fear that the use of the admin name could be a security issue, I would recommend you to use a two-factor plugin https://wordpress.org/plugins/two-factor/