Interesting problem. I replaced Windows 7 with Windows 8, using a valid product key. Several attempts to rid my computer of the windows.old folder failed. While working on removing that folder, I attempted to install the Windows 8.1 upgrade, but it was not in the Microsoft Store. Microsoft tech support suggested the Windows 8.1 would not install due to multiple error files in the log file. (?!) Those were cleared, and still, no Windows 8.1 in the store. Eventually, I was able to remove the windows.old folder, using Disk Cleanup. When the computer was running again after a reboot, the Windows 8.1 icon suddenly appeared in the Microsoft Store. Apparently, any trace of an OS other than Windows 8, will lock out the upgrade. Too bad MS tech support has no clue.