What should be done with old domain controllers after the upgrade is complete?

Demoting domain controllers that are running older versions of the server is an important process after an upgrade. This is because older domain controllers can cause compatibility issues and affect the overall health of the Active Directory environment. Demotion involves removing the domain controller from the domain in a controlled manner, which safely transfers any roles and data to the upgraded domain controllers while ensuring that the Active Directory remains stable and functional.

Once a domain controller is demoted, it no longer participates in domain operations and can no longer serve clients or replicate data. This helps to streamline the network environment, as keeping outdated domain controllers operational can lead to unnecessary complexity and potential security vulnerabilities.

Keeping old domain controllers as backups or repurposing them may seem beneficial, but it can introduce risks and complications if they are still part of the Active Directory infrastructure. Shutting them down without demotion could also lead to issues, as they may still hold important roles or data that other domain controllers rely on. Thus, demotion is the proper step to ensure the integrity and efficiency of your domain after an upgrade.