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License keys for Windows 7, 8, and 8.1 can be used to activate a fresh copy of the Windows 10 Creators Update, even if Microsoft has "officially" stopped offering free upgrades to Windows 10 back in the summer of 2016.
This means that users who want to update from Windows 7, 8, and 8.1 to the latest Windows 10 version don't need to buy new license keys to activate their new Windows OS.
All they have to do is install a fresh copy of the Windows 10 Creators Update, enter their old license key, and enjoy their new OS.
Upgrade mechanism continued to work past June 2016
When Microsoft launched Windows 10, it allowed users of older Windows versions a one-year grace period to upgrade to Windows 10 for free.
When that timeframe closed, users found that Microsoft didn't disable this feature, and they were still able to perform clean Windows 10 installs and use older license keys to activate Windows 10.
Microsoft didn't block this mechanism with the Anniversary Update last summer and didn't shut it down last week with the release of the Creators Update.
Trick used to cut costs when updating to Windows 10
Windows users have used this trick to save money when updating to Windows 10. Users buy cheap Windows 7, 8, or 8.1 licenses from various online stores, usually ranging between $10 and $40, instead of the more pricey Windows 10 product keys that can vary in price between $119 and $199.
This tactic is quite popular, but Microsoft has not moved in to shut down the activation of older product keys that have not been updated to Windows 10 in the one-year timeframe after Windows 10's release.
At this point, Microsoft is probably more happy about people still being interested in updating to Windows 10, an operating system for which the company has taken a lot of heat, especially for its intrusive user telemetry gathering, which is currently under scrutiny in the EU.
But as Reddit users have commented on this matter, this doesn't mean this is a completely legal way to get Windows 10, nor does it mean that Microsoft won't block this feature in the future.