Hello edpapp,
To my knowledge, all of the Windows Volume Licensing programs provide upgrade licenses, which means that a computer having a Volume License installed must first have an original Windows license, either OEM or retail, which is the original or base license
that is then being upgraded to the Volume License.
In much of its webpage messaging regarding licensing, Microsoft repeatedly states that the most cost effective way to get Windows licenses on computers is from the OEM.
However, for a purchase of such a large number, we would want to be absolutely sure, so my advice would be to contact a Windows Volume Licensing specialist. Please look for the contact info on the right side of this page:
www.microsoft.com/licensing If your organization is big enough to need 20,000 computers, it probably already has one or more contacts at Microsoft for software licensing, so maybe that would be a good place
to start.
Post back and let us know how it all turns out :-)
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