As with earlier downgrade rights, the customer is responsible for obtaining the installation media for the older operating system.

"Neither the manufacturer or installer, nor Microsoft, is obligated to supply earlier versions to you," read the licensing agreement. "You must obtain the earlier version separately."

However, unless Microsoft changes policies, OEMs will be able to offer new Windows 8 Pro PCs that are downgraded to, for example, Windows 7 Professional, at the factory. Computer makers will also be able to continue to sell Windows 7-powered PCs for up to two years after the debut of Windows 8 -- in other words, until late Oct. 2014.

For the latter, customers who later want to upgrade to Windows 8 must pay for the upgrade. That's not the case with a PC purchased with Windows 8 Pro that has been downgraded to Windows 7 Professional (or Vista Business).

"At any time, you may replace an earlier version with Windows 8 Pro," read Microsoft's licensing agreement.