I have a Mac Air. By using Boot Camp I managed to install Windows 8.1 Home x 64 bit by using an external drive

into my second partition. Meaning I have the option to use Mac operating system or Windows.

My Windows 8.1 is a Genuine Copy. Everything works fine.

Lately I managed to get a free upgrade to Windows 10 via internet. Everything also works fine.

Assuming six months later I don't like Windows 10, or my computer crashed or corrupted.

Now the question is this. Can I use my Genuine Copy of Windows 8.1 to re-install back into my Mac ?

If so, will it be activated again ?

Please kindly advice. Much appreciated. Thanking you all in advance.

By using the free upgrade to Windows 10 doesn't effect the product registration of your previous version of Windows unless you change the hardware that was used to register the previous version. You should have no problem reactivating Windows 8.1 when using the installation DVD you have for Windows 8.1.

I used the free upgrade offer with Windows 7 to go to Windows 10. Later I reinstalled Windows 7 from my installation DVD. By using my Windows 7 product key, Windows 7 activated just fine.

With the free upgrade, I thought there was a 30-day window to revert from Windows 10 to a previous version? Or is the 30-day period simply the time span where the files needed to revert (versus reinstall) to the old version are kept on the users hard drive?

When a Windows 10 upgrade initiates, it creates temp files of the previous Windows OS that it will keep for 30 days for use in reverting (rolling back) to the previous OS version. After the 30 days, Windows 10 will delete this temp directory.

If you still within the 30 days, you can chose to rename this folder/directory to prevent Windows 10 from deleting the files. This would preserve you ability to 'revert' or 'roll back'.

The preferred method would be to "Image" the Windows 7/8/8.1 partitions before upgrading. And "Image" the partitions again after a successful upgrade. Then use the Imaging software to revert back to whichever Image of the OS you wish to use.

As an aside, if I partition Windows for using Microsoft Office on my late 2013 iMac, do I open up my Mac to the usual Windows vulnerabilities? I think the answer is yes, but would be glad to hear your thoughts. Many thanks.