The licensing for Windows 10 when an upgrade from Windows 7 or 8/8.1 is done is that you have a legal license for Windows 10 on that machine or whichever version of Windows you upgraded from on that machine, but not both at the same time when the Windows 10 license is the result of an upgrade. [But keep your license key for the earlier version of Windows in your records somewhere. If you don't know it then snag it with Belarc Advisor or similar.]
Were you to wish to have both at once (dual boot) you would either need to buy a license for the earlier version of Windows and install it alongside Windows 10 [which is running under your upgrade license], or get a license for Windows 10 [brand new license] and install it alongside the earlier version of Windows.
You can perpetually wipe the drive and reinstall Windows 10, without even knowing the license key, on any machine on which Windows 10 was previously installed and legally activated. That's the beauty of this information being kept on Microsoft's servers and queried during the install itself. Were you not to have internet connectivity I believe the install will proceed, but there is a limited time that Windows 10 will continue to run unless it can "phone home" to determine it is legally licensed for that particular hardware.
If you start out with the Pro (or higher) version of the earlier Windows you will automatically get Pro when you upgrade. The digital entitlement records this fact as well, so subsequent wipes and reinstalls on Win10 will get you Win10 Pro.
Edited by britechguy, 30 December 2017 - 08:15 PM.
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Presenting the willfully ignorant with facts is the very definition of casting pearls before swine.
~ Brian Vogel