I've posted at length on the licensing issues in the past, quoting Microsoft "chapter and verse."
The long and the short of it is that Mr. Bennett is correct.
The "trick" here comes in having a Windows 10 license that was created using an earlier version of Windows as its licensing base. If that's the case you own a legal license to run your earlier version, but not Windows 10 at the same time, or Windows 10, but not your earlier version at the same time. In other words, it's perfectly legal to run one or the other, but you may not legally dual boot. If you wish to dual boot then you need to buy another license key for either the earlier version of Windows or for Windows 10 so that each thing you're booting has a separate license key (which is pretty much standard - this free upgrade, which was a first for Microsoft, was the first time where you could have a legally licensed version of a newer version of Windows where that key was generated using an existing license key as its basis. That's what threw the "monkey wrench" into the mix.)
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