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As long as I can remember, when you purchase a MS product license, you also are granted the right to downgrade to any older (supported) version of the product, but as long as you keep within the same product line. I.e.: if you buy Windows 7 Home, you'd only have right to downgrade to a Home product, either Vista Home or XP Home, not to a Vista Business or XP Pro. The same principle applies either for OS and for Office products. On the other hand, since you'd be buying an "upgrade-license", you'd need to make sure you get upgrade media for the product you intend to use, either the licensed version or any older one. So If you intend to purchase an upgrade for Win 8 Home, I think you wouldn't expect to be able to run a Pro or a Business version, no matter how older the downgraded Windows version be (let alone the fact that installation media doesn't allow you to crossover product lines when upgrading).