If you need to upgrade a server running Windows Server Standard to either the Enterprise, or Datacenter editions, it’s possible to do so online, without re-installing Windows.
Open an elevated command prompt and type DISM /Online /Get-CurrentEdition. This will return the current Windows version.
Type DISM /Online /Get-TargetEditions to list the Windows editions to which this server can be upgraded.
If you type DISM /Online /Set-Edition:ServerDataCenter you’ll get the message in the screenshot below. This is because even if you’re using a KMS server for internal activation, you have to provide a product key. Fortunately, Microsoft have a page that lists the KMS client setup keys http://technet.microsoft.com/en-us/library/ff793421.aspx. On this page you can find keys for multiple Windows Server versions.
Typing DISM /Online /Set-Edition:ServerDataCenter /ProductKey:xxxxxx will upgrade the operating system. All that’s required to complete the upgrade is a reboot.