If you have purchased Windows Server 2008 via volume licensing then the process for downgrading is simple. Contact Microsoft, and they will provide you with a download of the media and a license key to use.
If you have purchased Windows Server 2008 as retail product or OEM then the process is more complex as Microsoft will not provide you with the media or a license key. MS basically indicated that you have the right to downgrade but they do not provide the means. You can perform the downgrade by:
- Obtaining an OEM key and media from any source. The source could be another server you own or another company or your OEM vendor. You do not need to own the OEM key and media used.
- Obtaining a retail key and media from any source. Again, you do not need to own the retail key or media, just obtain and reuse it.
- Note that you are not allowed to use the volume license key of another oganization. Only your own volume license key.
When purchasing servers for this reason alone, I would consider volume licensing over OEM versions of Windows Server. It may be a few dollars more (not much), but you gain flexibility. As well, you should consider that OEM versions are tied to the specific hardware you bought it on and cannot be migrated to new hardware. A new OEM version must be purchased for new hardware.