Once subscribed to Identity Manager you cannot:

• edit account details from /admin/users
• edit account details from id.atlassian.com

Message Displayed when attempting to edit account details in id.atlassian.com:

Your account is managed by your organization. Contact your organization administration to change your email address

According to our documentation

You can change someone's username, name, and email address for their account by editing the user from the user management page.

We should make it clear that when subscribed to Identity Manager, editing user details is done through Organization > Managed Accounts