This article will cover demoting of Windows Server 2008 DC server after Windows Server 2012 R2 is added to domain as DC.
Edit August 2021 – we are also almost at the point (October 2023) when we will have to discontinue our Windows Server 2012 R2 DCs – I put up guide on how to demote Windows Server 2012 R2 and you can follow it here – https://www.informaticar.net/demote-windows-server-2012-r2-dc-frs-to-dfs-migration-is-also-covered/