Found the info below last night while discussing support terms for M$ OS's - Figured some other people here may find it helpful/interesting
Every Windows product has a lifecycle. The lifecycle begins when a product is released and ends when it's no longer supported or sold. Knowing key dates in this lifecycle helps you make informed decisions about when to upgrade or make other changes to your software. Here are the rights and limits of the Windows lifecycle.
End of sales
End of sales refers to the date when a particular version of Windows is no longer shipped to retailers or Original Equipment Manufacturers (OEMs). Examples of OEMS are Dell and Toshiba—PC manufacturers who often preinstall Windows software. When a version of Windows reaches its end of sales date, it's a good time to think about upgrading...
Windows Downgrade Rights
To use prior versions of Windows software on PCs installed with newer versions, it is possible for consumers to obtain a license for Downgrade Rights. The following table compares Volume Licensing Downgrade Rights with those provided under OEM and FPP licenses...