Windows Server 2008 R2 Standard to Enterprise Upgrade

[sgunelius]

We have some ProLiant BL460 G8 servers with 32GB memory running Windows Server 2008 R2 Standard that now need 128GB to accommodate the applications already installed and configured, which means we'll have to go to Enterprise to utilize the additional memory.  I'd normally perform a fresh install of Enterprise edition, but considerable time, effort and money has already been spent installing/configuring the existing applications on these servers, so I'm wondering what options I might have for an in-place upgrade.

Has anyone used Microsoft’s command-line utility, DISM (Deployment Image Servicing and Management) to perform an online upgrade?  From what I’ve read in the URL below, the components for upgrade are already present on the existing OS, so no media should be required when using DISM, but we will have to purchase and register the Enterprise licenses using the following syntax (DISM /online /Set-Edition:ServerEnterprise /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX).

http://blogs.technet.com/b/server_core/archive/2009/10/14/upgrading-windows-server-2008-r2-without-media.aspx

On a related note, we purchased the original OS licenses through HP (got the product key tag on the servers), but from my experience the installed OS from HP's Windows Server 2008 R2 Standard DVD shipped with the server isn't necessarily the same as the retail media from Microsoft.  If I obtain Enterprise licenses through retail channel, will those license keys work on this install?