Clean installs can be performed after free Windows 10 upgrade

I still don't understand why Windows 7 AND Windows 8 get a free upgrade, yet without either or it'll cost me $200 to pick up a Windows 10 Pro license when I can buy a refurb Windows 7 computer for $100...Will the product key for that said Windows 7 computer then no longer work after I use the Windows 10 upgrade?

Simple. Windows 7 accounts for nearly 60% of the desktop OS market share. They need and want that huge chunk on Windows 10 ASAP, and well, it wouldn't be fair to NOT give it to Windows 8 users as well.
 
I have a i7 desktop running Win 7 Pro with a license for refurbished pc. I want to install a ssd drive with a clean install of the OS. They didn't provide a cd of the os when I purchased it. I'm thinking perhaps I can re-install Win 10 from an iso after the upgrade ??? Not really clear what the best approach would be ... any recommendations?
 
This is a question. Can I do a clean install of Windows 10 64bit if I upgraded from Win 7 32bit, but I owned the Win 7_64 bit license too?
 
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