Microsoft will allow users of Windows 8 Pro to downgrade their new PCs to Windows 7 or even Vista, according to the operating system's licensing agreement.

Not surprisingly, users may not downgrade to the still-used-but-slated-for-retirement Windows XP.

Downgrade rights -- which let customers replace a newer version of Windows with an older edition without paying for two copies -- are available only in Windows 8 Pro. That fits with previous practice: Only Windows 7 Professional, for instance, was allocated downgrade rights.

"Instead of using the Windows 8 Pro software, you may use one of the following earlier versions: Windows 7 Professional or Windows Vista Business," states the software license agreement for the version of Windows 8 Pro that will be installed by computer makers (OEMs) on new PCs.

Windows XP Professional, which was one of the allowed downgrades for Windows 7 Professional, was not named. Windows XP will fall off Microsoft's support list in April 2014.

One licensing expert noticed XP was AWOL.

"So no downgrade rights to XP. Also note that the soon-to-come Office 2013 will not support XP. So we can see they are trying to strangle the life out of XP," said Kenny Chan, a technology specialist for CDW, in message earlier this month on a LinkedIn thread dedicated to Microsoft licensing professionals.

"What I don't know is how long Microsoft will keep XP available for download on VLSC for volume licensing customers after the debut of Windows 8," Chan added. VLSC (Volume License Service Center) is the online portal for companies that have signed volume licensing agreements with Microsoft.

Unlike consumers or small businesses, corporations armed with enterprise licensing agreements, including the annuity-like Software Assurance, are allowed to downgrade from any version of Windows to any previous edition.

Downgrade rights became noteworthy after Windows Vista's 2007 launch when many users, frustrated at that edition's problems, mutinied and dropped back to XP.

Traditionally, downgrade rights are available only from OEM copies of Windows, those that are pre-installed by computer manufacturers. It looks to be the same with Windows 8: The software license for the retail version of Windows 8 Pro omitted the section on downgrades.

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.

Do-it-yourself downgrades will be more complex with Windows 8, however, as users must first modify the PC's BIOS to boot into what's called "legacy mode." By default, Windows 8 will use UEFI-mode (Unified Extensible Firmware Interface) boot on new PCs to enable some new features, including Secure Boot.

Notebooks, desktops and other devices powered by Windows 8 Pro will go on sale Oct. 26.

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, on Google+ or subscribe to Gregg's RSS feed . His email address is gkeizer@computerworld.com.

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