Windows 7 activation error: invalid product key
http://pcsupport.about.com/od/osprod...t-keys-faq.htmYou might be using a product key that doesn't match the version of Windows 7 that's installed on your computer. Check the product key to make sure it is for the version that you are trying to activate.
Nor can you upgrade Windows 7 Professional to Windows 10 Home for free (and I don't think you can do it even with a purchased Windows 10 Home Product Key):"Can I use the product key from one version of Windows to install a different version of Windows?"
No, you can not. Windows product keys are exclusively associated with a particular version of Windows.
For example, you can not install Windows 7 Home Premium with a Windows 7 Professional product key, nor can you install Windows Vista Business with a Windows 7 Ultimate product key.
You can only use your product key for the specific Windows operating system version for which it is coded.
Windows 10 Specifications - Microsoft
Activation in Windows 10 - Windows HelpWe’ll keep you on like-to-like editions of Windows. For instance, if you are using Windows 7 Home Premium, you’ll upgrade to Windows 10 Home.
But we sincerely hope you get $49 worth of enjoyment out of trying!How you activate Windows 10 after reinstalling it depends on how Windows 10 was originally installed on your device.
If you activated your free upgrade to Windows 10 or bought and activated Windows 10 from the Windows Store, you have a digital entitlement for your device. This means you can reinstall the same edition of Windows 10 that your device has a digital entitlement for without entering a product key. During reinstallation, if you’re asked to enter a product key, select Skip. Your device will automatically activate online after the installation is complete.
So one of the PCs was not activated, or in 30-day grace period? That's a different story then.
If you run the "upgrade this PC now" option of the MCT, it will automatically pick PRO. If you attempt to run Windows 10 Home setup.exe to upgrade from Windows 10 PRO, it will ask you for a product key, and not having a valid product key, will then refuse to activate. Betcha' a doughnut that is exactly what happens. But it's your money and your time.