In their Windows Server 2008 R2 Upgrade Paths document, Microsoft indicates the following about cross-edition upgrades:
Cross-edition upgrades (for example, the Windows Server 2008 Foundation SKU to the Windows Server 2008 Datacenter SKU) are not supported.
However, it looks as though dism can be used to upgrade editions. See this Technet blog post for more details. Please note that the blog indicates this upgrade cannot be performed on a Domain Controller.
Display current edition:
PS C:\> dism /online /get-currentedition
Deployment Image Servicing and Management tool
Version: 6.1.7600.16385
Image Version: 6.1.7600.16385
Current edition is:
Current Edition : ServerStandard
The operation completed successfully.
Check the available editions:
PS C:\> dism /online /get-targeteditions
Deployment Image Servicing and Management tool
Version: 6.1.7600.16385
Image Version: 6.1.7600.16385
Editions that can be upgraded to:
Target Edition : ServerDataCenter
Target Edition : ServerEnterprise
The operation completed successfully.
Upgrade to Enterprise:
PS C:\> dism /online /Set-Edition:ServerEnterprise ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX
...
