But he did have one caveat. "Keep in mind that this is not supported, and could end up with a catastrophic failure if some bits are not properly upgraded," said Fay. "But 12 hours in, and things seem stable."

Cherry said his upgrades were identical to those promised by Microsoft for Windows 7 users. "It's a traditional upgrade [that retains] all the settings and applications," he said. The only glitch he encountered was with Cisco's VPN Client, which required reinstallation on Windows 8 RTM.

Changing the numbers in one file of the Windows 8 RTM installer lets users upgrade from Windows 8's free previews.

Although Cherry did not test the upgrade trick with a copy of the , he expected that would work just as smoothly as to a perpetual license of Windows 8, like those Microsoft released last week to TechNet and MSDN subscribers, and to enterprises with Software Assurance plans in place.

Because the Windows 8 previews will expire on Jan. 15, 2013, users who want the longest free ride can wait out the next five months with Release Preview, then -- assuming the free eval is still available -- upgrade to the trial next January for three more months of use.