Let's have a look how to do that. Just remember, it's probably not a valid downgrade, but it works perfectly. As long you have a valid product key, nothing illegally is done.
The following Windows versions can upgrade free to Windows 10 Home: Windows 7 Starter, Windows 7 Home Basic, Windows 7 Home Premium and Windows 8.1 editions
The following Windows versions can upgrade free to Windows 10 Pro: Windows 7 Professional, Windows 7 Ultimate, Windows 8.1 Pro and Windows 8.1 Pro For Students editions
The following Windows versions can upgrade free to Windows 10 Mobile: Windows Phone 8.1 only
1. Open regedit.exe and navigate to HKLM\Software\Microsoft\Windows NT\CurrentVersion
2. Change ProductName to Windows 8.1 Professional
3. Change EditionID to Professional
4. Navigate now to HKLM\Software\Wow6432Node\Microsoft\Windows NT\CurrentVersion
5. Change ProductName to Windows 8.1 Professional
6. Change EditionID to Professional
7. Close regedit.exe (no need to restart)
8. Start the Windows 8.1 Pro installation
With that you're done and a free upgrade to Windows 10 Pro is possible! Features missing are Enterprise options like Direct Access and language packs, but I don't miss them on my home PC. Happy upgrading to Windows 10 :-)
Note: This guide will work for Windows 7 and 8 as well.
Update 6-7: When people may experience issues when downgrading, maybe this will help? Windows 7 Downgrader: http://answers.microsoft.com/en-us/windows/forum/windows_7-windows_install/downgrade-win-7-ultimate-to-win-7-professional/cfebbc77-683a-4d32-bf3a-e897619a82f7
Otherwise it's possible to become an Windows Insider too (without the need to have an official product key): http://blogs.windows.com/bloggingwindows/2015/06/19/upcoming-changes-to-windows-10-insider-preview-builds/
Update 2-8: Joe Donkor mentions:
I ran through this today. I just couldn't get step 8 to work and got the message that the computer was being rolled back to the previous configuration. In desperation after a few tries, I just ran the Win 10 pro install and it worked without issue. So if you get stuck at the same point I did, give it a try. > Just great to hear that! Good try :)
