Windows 10 users who have not updated their Insider preview since early July have until Thursday to get the latest code or face staring at a blank screen because the PC won't boot.

All builds issued between Jan. 23 and July 9 -- there were 11 builds during that period, numbered 9826 through 10166 -- will refuse to boot after Thursday, Oct. 15, according to a revised message posted by a Microsoft support engineer on the company's discussion forum.

Build 10166 was the last one issued by Microsoft before it rolled out the production code, dubbed RTM for "release to manufacturing," on July 29. As it approached RTM, Microsoft temporarily stopped releasing Insider builds. It resumed them on Aug. 18.

"If you are running a Windows 10 Insider Preview Build 10041 through 10166, you have a short window where you can perform an upgrade to the current build, however to do this, you must take action before October 15, 2015," the engineer, identified only as "RajithR," wrote last week.

The early-build shutoff ensures that testers and Microsoft do not waste time filing and reviewing bugs that have already been fixed in older previews.

The deadline was not out of the blue: Personal computers running the 11 builds -- RajithR's claim that the run started with build 10041 rather than 9826 contradicted a chart he also posted -- have displayed warnings of the impending expiration since mid-September. Starting Oct. 1, the PCs have been rebooting every three hours, another not-so-subtle hint to update.

If users ignore the warning, then try to recover use of their devices after Thursday, they will need to boot from physical media and again opt in to the testing program. In that case, the Windows 10 Insider build used to power the PC will not be activated, a term Microsoft uses to describe a legitimate license.

In his message, RajithR also spelled out a multiple-step process for the laggards who must boot from a USB drive or DVD disc; that drive or disc can be created after downloading a Windows 10 .iso disk image from Microsoft's website.

Microsoft issued similar warnings in April about the three Windows 10 preview builds released in 2014.

The Insider program, its rolling deadlines to stay up-to-date, and their impact are important in part because of Microsoft's policy, while not explicit, to let anyone run the preview -- including those who did not upgrade from an eligible and legitimate license of Windows 7 or 8.1.

After much back and forth earlier this year -- first it seemed that Microsoft would provide a one-time amnesty to pirates and let them upgrade to Windows 10 from counterfeit copies of Windows 7 and 8.1 -- the Redmond, Wash. company let users who had adopted the Insider track continue running Windows 10 Insider as long as they installed it before July 9 and continued running the preview. That includes Insiders who installed from an .iso disk image to a virtual machine.

Even so, Microsoft made it clear that it considered the practice outside legality's bounds. "Windows 10, whether you get it on 7/29 or whether you got it in a preview form through the Windows Insider Program, is intended to be installed on [a] Genuine Windows device," said Gabriel Aul, engineering general manager for Microsoft's operating system, in a June 22 blog.

People who have Insider installed without an underlying linked license to Windows 7 or 8.1 should be especially interested in maintaining the OS for their testing: Once they drop off the preview program, they may reinstall Windows 10 and shift to Insider, but as RajithR said, the copy will not be activated.

Users can check the Insider build's expiration date on their device by typing winver.exe in the Windows 10 search bar. For example, the latest preview, build 10565, which was released Monday, expires on July 16, 2016.