How Apple notifies users whose Apple ID conflicts with Apple School Manager
Apple will identify users whose Apple ID contains the domain name you’ve registered. Those users receive a mail message and a notification telling them they must rename their Apple ID. If they’ve not renamed their Apple ID yet, they keep receiving notifications and mail messages, with one final mail message at 60 days. After 60 days, the user’s Apple ID will be automatically renamed to a temporary username, and the original Apple ID is released and claimed by your organization.