If you have a need to upgrade a server running Windows Server 2008 R2 Standard to either the Enterprise, or Datacenter edition, it’s possible to do so online, without re-installing Windows. This not only a simple process, it saves you time.

Open an elevated command prompt and type DISM /Online /Get-CurrentEdition. This returns the current Windows version.

Type DISM /Online /Get-TargetEditions to list the Windows editions to which this server can be upgraded to.

Type DISM /Online /Set-Edition:ServerDataCenter /ProductKey:xxxxxx will upgrade the operating system. All that’s required to complete the upgrade is a reboot.

Done!

Ref Info:

DISM Windows Edition-Servicing Command-Line Options – https://technet.microsoft.com/en-us/library/hh825157.aspx

KMS Client Setup Keys – Link