Just a day before Microsoft Corp. drops support for Windows XP Service Pack 2 (SP2), the company announced on Monday that people running some versions of Windows 7 can "downgrade" to the aged operating system for up to 10 years.

The move is highly unusual. In the past, Microsoft has terminated downgrade rights -- which let customers replace a newer version of Windows with an older edition without paying for two copies -- within months of introducing a new operating system.

While few consumers may want to downgrade from Windows 7 to XP -- unlike when many mutinied against Vista three years ago -- businesses often want to standardize on a single operating system to simplify machine management.

Monday's announcement was the second Windows XP downgrade rights extension. Microsoft originally limited Windows 7-to-Windows XP downgrades to six months after Windows 7's release, but it backtracked in June 2009 after a Gartner Inc. analyst called the plan a "real mess."

Instead, Microsoft later said it would allow downgrades to Windows XP until 18 months after the October 2009 debut of Windows 7, or until it released Windows 7 SP1.

In either scenario, XP downgrade rights would have expired sometime in 2011, perhaps as early as April.

On Monday, Microsoft again changed its mind. Users running Windows 7 Professional or Ultimate will now be able to downgrade to Windows XP Professional throughout the entire life cycle of Windows 7.

"Our business customers have told us that the removing end-user downgrade rights to Windows XP Professional could be confusing," said Microsoft spokesman Brandon LeBlanc, in an entry on the company blog.

Windows 7 Professional won't be fully retired until January 2020; the Ultimate edition will be put out to pasture five years earlier, in January 2015.

Although Microsoft said it made the change to simplify the work in tracking licensing rights for PCs, the continued popularity of Windows XP may have had something to do with it. At Microsoft's Worldwide Partner Conference (WPC), which opened Monday in Washington, D.C., a company executive acknowledged that 74% of business computers still run XP.

The downgrade rights are available only from OEM copies of Windows 7, those that are pre-installed by computer makers.