New
#11
You really should stop inserting hidden meaning into what's being said. Stop looking for what wasn't said, and concentrate on what IS said.
What he means is simply that Windows 10 will automatically detect the already installed Windows key. You won't have to re-enter it, and thus Windows 10 will activate without any prompting.
"once it has been successfully upgraded to Windows 10" just means "after the upgrade install has completed, it will re-activate using the already installed key". It doesn't mean that anything happens to your old key.
Technically, yes... you could probably re-use that license on another computer and it would likely work. Legally, however, that would not be allowed as you are upgrading that license. You could always re-install Windows 8.1 on that same computer, if you delete Windows 10. Or, if it's a retail copy of Windows 8.1, you could delete Windows 10 from the computer and then Install Windows 8.1 on a different computer, but not if it's an OEM copy (the key is embedded in the BIOS anyways in those cases).