I successfully installed Windows 10 in my Parallels 9 VM, including Parallels Tools. All this was done via legitimate means:
1) Make sure you've previously managed to reserve an upgrade.
2) Uninstall Parallels Tools from your Windows 7/8/8.1 VM.
3) On your Mac, visit the Microsoft Windows 10 ISO download page
and download the edition for you.
4) Once downloaded, mount the ISO in your Windows VM using the standard means (Devices > CD/DVD > Connect Image in Parallels 9).
5) Launch setup from the mounted image and follow the instructions to upgrade to Windows 10. DO NOT PERFORM A CLEAN INSTALLATION OTHERWISE YOU'LL LOSE YOUR FREE UPGRADE.
6) Windows will upgrade itself to Windows 10.
7) Follow through the setup steps at the end. Once you're back to the desktop, you should find Windows 10 is activated.
8) Follow the "Scenario 2: More Complicated" steps on the Parallels Tools manual installation
to get Parallels Tools reinstalled.
Note that on Parallels 9 at least, shared folders (\\psf in Windows) doesn't appear to work. Coherence is a bit janky too. Everything else works fine.
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