How can I upgrade from Windows Server 2008 R2 Standard to Enterprise Edition?

Our IT guy mistakenly set up Windows 2008 R2 Standard Edition instead of Enterprise Edition. The machine has 64GB of RAM, and Standard only supports 32GB, so half of it is unusable.

Is there a way to upgrade to Enterprise Edition without a complete re-install? He has already generated some CSRs for SSL certs and done other work we'd hate to waste. (The cost of the license isn't an issue, just mechanically how to best perform the upgrade.)

Windows Server 2008 R2 includes a command-line tool called the Deployment Image Servicing and Management tool, or DISM. It's functionally equivalent to the Windows Anytime Upgrade feature in client versions of Windows. You can run it from an elevated command prompt.

To determine the currently installed edition:

DISM /online /Get-CurrentEdition

To determine the editions you can upgrade to:

DISM /online /Get-TargetEditions

To actually perform an upgrade:

DISM /online /Set-Edition:<edition ID> /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX

Source (with more information): http://blogs.technet.com/b/server_core/archive/2009/10/14/upgrading-windows-server-2008-r2-without-media.aspx