Can I upgrade to Win10 and swap drives with Win7?

[brahman]

Hi,

I am still not decided if I want to upgrade to Win10, but time is running out.

If I will put in a brand new hard drive and upgrade to Win10 on this new 2nd hard drive with USB Win10 Install media created by the Media Creation Tool and give my Lenovo OEM Win7 Pro Product key to activate this Win10 installation, and let's say after 2 months I decide I want to stay with Win7, can I just plop in the old Win7 drive and be good?

Can I switch back and forth on the same machine this way?

Or is somehow magically my old Win7 installation being deactivated by MS?

[Ath]

Legally you can't use your Win7 installation in parallel with your Win10 installation. Technically though, you can.
You are allowed to revert your Win10 installation to Win7 (or Win8.1 if your upgraded from there), within the first 30 days after the upgrade. And you can again upgrade to Win10 later if you so desire, even after the free upgrade period has passed, as the license has already been registered at MS to be eligible for upgrade.
I haven't tried to do a fresh install with a Win7 license that was already upgraded to Win10, (and I don't intend to, ATM) but you can ask google what would/could happen then... I suspect the same legal issue as mentioned above.

[brahman]

you can again upgrade to Win10 later if you so desire, even after the free upgrade period has passed, as the license has already been registered at MS to be eligible for upgrade.

Are you 100% sure about this?

What about my old drive: Will the Win7 activation be deactivated in some way, when I upgrade to Win10 after 30 days, or can I still swap drives and switch back to Win7 after a few months?

[xtabber]

You should make a backup copy of your Win7 activation tokens beforehand.

See this post for more on this topic.

[brahman]

You should make a backup copy of your Win7 activation tokens beforehand.

Thanks,
I already did a while ago.