Should I purchase Windows 10 upgrade disk?

I've got a retail Windows 7 disk with a key, and I've received Microsoft's free upgrade to windows 10 (that from what I've understood by checking my hardware)... I'm thinking about buying a new PC in a month or two, and since the windows 10 activation won't work on my PC (as it has new hardware), can I buy windows 10 upgrade, and upgrade my windows 7 into 10? I mean, even if I've used Microsoft's free upgrade? I'm wondering if it'd like, put my windows 7 key into a "used" list or something...

• I will format the current PC, so there won't be a collision or something...

TL;DR : Can I buy Windows 10 upgrade from 7 and use it in a different PC, even if I've already gotten Microsoft's free upgrade?

Thanks!

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