Online Documentation on Managing Domains is Incorrect

When a server is promoted to primary domain controller, an up-to-date copy of the user account database is replicated from the old primary domain controller to the new one, and then the old primary domain controller is automatically demoted to backup domain controller (unless it is not currently running).

Usually, when a server is promoted to primary domain controller, no special action needs to be taken to demote the former primary to backup domain controller because the system does this automatically.

However, if a server is promoted to primary domain controller while the existing primary domain controller is unavailable (for example, if it is being repaired), and the former primary domain controller later returns to service, you must demote it.