So if you have a Win7 install...and you decide to upgrade to 10...and decide you don't like it...does it mean you can't go back to 7 ? ?
Upgrade versions have been available since Windows 95. Windows 10's upgrade version being offered for free is unlikely to be inconsistent with their former upgrade versions.
"Upgrade" Licenses include as part of their terms that you already have a license for an applicable previous version, and as part of the terms of the new version, that old one is invalidated.
For example, if you had Windows 95 Installed and bought a Windows 98 upgrade, that Windows 98 Upgrade's License, as part of it's terms, requires that you effectively yield a license for an applicable piece of software. In the case of Windows 98 Upgrade, it requires Windows 3.1 or Windows 95 to be yielded. However, this is only in effect as long as the Windows 98 Upgrade is installed. If you uninstall it, than the license is no longer in use and you can freely use the licenses that you had yielded while it was in use. Note that if you get a Full copy of Windows 98 and upgrade Windows 95, that Windows 95 License is still completely valid- it is only Upgrade Editions of Windows 95,98, ME, XP, Vista, etc. that require you to yield an applicable product's license in order to make use of the new one- upgrading with a full retail copy using the "Upgrade" option does "free up" the license for the older version- if you use a retail copy of Windows XP to upgrade Windows 98, for example, than that Windows 98 License is "free" to be used again.
The WIndows 10 "upgrade" is an "upgrade" version, however- and I find it doubtful that they will change the rules regarding upgrade versions significantly for Windows 10. Therefore, in your scenario, it would be exactly like it would have played out with any previous version of Windows and an applicable upgrade.
If you have Windows 7, and upgrade to Windows 10, part of the upgrade requires you to have an applicable product, as well as yield the license for the product.
If you uninstall Windows 10, that "yielded" license can be used again. What you cannot do (if you want to adhere to the EULA, anyway) is upgrade Win7 to Windows 10, and then use that Windows 7 license to install to another computer, because the Win10 upgrade "yields" that license of Win7.
Effectively the Free WIndows 10 upgrade is offering a free Windows 10 upgrade version to everybody for a year that has Windows 7 or 8. Once you "have" the upgrade, it will effectively be like having an "upgrade edition" that you can use at any time, as long as you yield an applicable software license while doing so.
With Windows 98 there was an upgrade option. You would upgrade Windows 95 to Windows 98 with a Windows 98 Upgrade CD.
The license of the Windows 98 upgrade version had as part of it's terms that you had a eligible copy of Windows 95, and that, further, the windows 98 Upgrade "replaced" it in terms of the license applicability. This meant that you couldn't upgrade a Windows 95 computer and then install that copy of Windows 95 on another computer- that license no longer "exists" because it is part of the Windows 98 install. (Of course, nothing stopped you from doing this)
You could uninstall Windows 98 and reinstall Windows 95, however. The Windows 98 License was tied to the Windows 95 License, but uninstalling Windows 98 freed that license.
Since this is how it has been done with upgrade versions since windows 95, I don't see why it would work any different with Windows 10. The only difference is the upgrade edition is being offered for free.
it is not unreasonable to conclude based on that, If you upgrade a Windows 7 system to Windows 10, you cannot reuse that Windows 7 license on another system unless you also uninstall Windows 10- same as the Win98 Upgrade- it is tied to the license of an applicable product in that part of the terms of the upgrade version include the invalidation of the license of the upgraded version for the duration of your use of the version being upgraded to. But once you uninstall Windows 10, that upgraded Windows 7 license is no longer invalidated because the Windows 10 EULA no longer applies, since it only invalidates the previous versions license when it is installed.