If you enable the setting again, users will immediately lose the ability to change their username on your site.
On each logon, the users’ name will be reverted to the one from the payload, if possible. Of course, this might not be possible (another user might already use that username), so users might still get suggested usernames that differ from the SSO one.
Assuming your SSO only provides username that are legal in Discourse and all user log in regularly, after some time, the usernames will all match again.