Can I use my Windows 10 key again if I decide to go back from Windows 7?

[Fornever]

Say I want to install Windows 7 (for personal reasons) activated with a product key I buy from eBay, over my current Windows 10 instance - which I activated a year ago with a freshly bought USB that comes with the product key printed on the card - and then if I decide to go back to 10, would my Win 10 product key work again after that?

 

[Dugimodo]

On the same hardware, yes it will. You may not even need to use the key again as MS activation servers remember the hardware.

If it's a retail copy of windows it can also be transferred to different hardware if you want. If you want even more security link the key to an online MS account, then your account will store the fact that you own a copy of windows for you.