When talking about Windows downgrade (Assume Windows Server), few issue you require to take concern. Some vendor will propose you Windows Server ROK (Reseller Option Kits), in short, ROK is another type of OEM edition; while some vendor will advice you for Windows open license which cost you higher compare to ROK.
In Microsoft licensing point of view, you may downgrade the server as long you own the the Windows license. Example Windows Server 2008 R2 64 Bit, you may downgrade to Windows Server 2008 32 bit or Windows Server 2008 to Windows Server 2003.
Why purchase Open License?
For Open License, you may login to Microsoft portal, list of allowed downgrade version will be shown there, together with the license key / activation key and the downloaded media.
Limitation of ROK / OEM license
Unfortunately, for OEM / ROK, you do not have this option. Legally, you allow to downgrade, but you require to obtain your own CD Media and Activation key. Vendor like IBM and HP will not provide you nor Microsoft them self. Personally I am not agree with Microsoft because they advice end user to borrow from other end user who own the version they want to downgrade. How many really willing to borrow the activation key?
More information about your Windows downgrade, check out Understand Windows Downgrade Rights. While to solve the ROM / OEM issue, in case you unable to locate a correct activation key and your evaluation period is expired, you may also use another method call windows “rearm“, I will write about how to apply this method in my following articles, it will be a good method to extend your windows evaluation period, and this method is not about crack or hack, it is a legal method which recognize by Microsoft.