Sometime ago, I wrote a blog about upgrading from Windows 2003 based Active Directory to Windows 2008 R2 based Active Directory. If you are planning to upgrade now from Windows Server 2008 R2 based Active Directory to Windows Server 2012 R2 based Active Directory, the steps are the same and you can found them at my earlier blog.
Step by step guide for upgrading Active Directory from Microsoft Windows 2003 to Microsoft Windows Server 2008
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