Presumably, this will change as we get closer to Windows 11's release date. I was also still able to opt in to the Windows 11 preview builds after reinstalling Windows 10 on the old XPS 13, albeit with the typical 'Your PC does not meet the minimum hardware requirements and there may be issues and bugs' warning message. Neither of the two unsupported PCs I'm running the Insider Preview on-a Dell Latitude 3379 with a Core i3-6100U and a Dell with a Core i5-4210U-have received this message yet, and they continue to download and install new updates on the Dev and Beta channels. ZDNet reports that some users of unsupported PCs are beginning to see a message telling them to reinstall Windows 10 when they attempt to enroll older PCs in the Insider program after performing a clean install of the Windows 11 preview. Open Windows Settings > Update & Security > Windows Insider Program.
Many older PCs have been happily running the Windows 11 previews for months now despite Microsoft's stringent official system requirements, but that's about to change. Andrew Cunningham reader comments 198 with
