Howto switch from Windows Server 2012 Standard Server to Windows Server 2012 Datacenter Server without having to reinstall

open an administrative power shell session and pop your new key in using the dism tool

dism /online /set-edition:ServerDatacenter /productkey:<Datacenter key, e.g. AAAAA-BBBBB-CCCCC-DDDDD-EEEEE> /AcceptEula

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http://community.spiceworks.com/how_to/show/30293-howto-switch-from-windows-server-2012-standard-server-to-windows-server-2012-datacenter-server-without-having-to-reinstall

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