I have tried to upgrade a laptop bearing a Windows 8.1 Single Language OS a while ago. I have used the ISO MICROSOFT.WINDOWS.10.PRO-CORE.RTM.10240.X64.OEMRET.ENGLISH.DVD-WZT to upgrade it. Same ISO have been used on the other laptop with Windows 8.1 Pro WMC. The Problem: The laptop bearing a Windows 8.1 Single Language OS (OEM activated) ask for a key when trying to upgrade (in-place). The laptop with Windows 8.1 Pro WMC upgraded successfully without needing a key. The laptop bearing a Windows 8.1 Single Language OS has been updated with latest updates already but I haven't seen any icon for the reservation of Windows 10. What could have been the problem? I am thinking of adding a feature to make the Windows 8.1 SL into Pro and then applying kms and then upgrading it to Windows 10 again. Will it work? Has anybody tried to Upgrade a Windows 8.1 activated via kms to Windows 10 and does not ask a key?