windows server 2003 r2 upgrade

How to upgrade WINDOWS SERVER 2003 ENTERPRISE EDITION SP 2 (NON-R2) to WINDOWS SERVER 2003 R2 SP2?
I have a Server where Windows Server 2003 SP2 (non-R2 version) is running. I want to upgrade it to R2 version of Windows Server 2003. How can I do that?
You cannot upgrade Enterprise to Standard (R2 or not). You would have to reinstall.
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Instalacion Windows Server 2003 R2