So you need to obtain a digital entitlement from Microsoft, that upgrades your Windows 7 license to Windows 10 license. It is bound to the computer hardware even in case you had a retail non-OEM version of Windows 7 purchased. I suppose you do not have any corporate licensing model applied (MPSA, Select...).
See Activation in Windows 10
Digital entitlement is a record stored on Microsft activation server that contains the hardware hash and the edition of Windows. Nothing else enters into it. Neither Microsoft account, nor the original product key.
Digital entitlement is created when you upgrade to Windows 10. It is possible to get it even with a clean install by entering the product key from the original version of Windows (if this has not been used recently). Without installing Windows 10 you can not get it.
But you can back up the current status (backup image), upgrade to Windows 10, and after activation you can roll back (via the backup image, or directly using the rollback button in Windows 10).
(Corrected previously given info:)
Information about HDD are stored in Hardware hash, but do have a low priority. The hardware hash is what you get by gatherosstate.exe tool. But when you try to activate using a different and pre-installed HDD, as suggested by @harrymc, you run into a risk, that your hardware hash will be different from what you have registered with Microsoft activation server, and you will not find the digital entitlement during activation and will have to buy the Win10 anyway, so all the efforts will be probably fruitless. Also running the same OS image on various computers is not supported by Microsoft. It forced me few times to phone activation in the past.
So one viable option for you is to upgrade, activate and then immediately roll back. If you are lucky, then no backup images will be necessary. So you can simplify the process you have suggested only by using built-in rollback instead of cloning back the image of the Win 7 disk. Of course the rollback can go wrong sometimes.
There is no special tool that would allow you to query the Microsoft activation server's database for your hardware hash except of the activation process itself.
(source link)
2yes, creating backups is the best solution. After the update to Win10 also create new images – magicandre1981 – 2016-06-10T15:20:52.180
And I guess the only way to keep the license is to backup it with Microsoft account, then you can ever reuse it by login again – Mahdi Rafatjah – 2016-06-13T11:16:08.783
Try to ask Microsoft about this, nobody else really knows how the
digital entitlementthing really works. – Vojtěch Dohnal – 2016-06-13T11:27:08.180@MahdiRafatjah entirely irrelevant. He doesn't want to stay on Win10 but keep the Win7 backups. The opposite – Dr.Ping – 2016-06-14T08:46:10.860
Yes, this method should do what you need. This is what I'd do in your situation. Kinda sad that MS cannot be trusted anymore – Dr.Ping – 2016-06-14T08:48:01.733
I just asked the same question here. It seems to me that using a spare disk for the installs would be less time consuming than making win7 backups on each machine.
– The111 – 2016-06-15T07:25:18.857No need for Microsoft account. Digital entitlement works with a hardware id. So after reinstalling 10 it will detect automatically if the computer has been upgraded to win 10 before, without any need of Microsoft account. – TJJ – 2016-06-15T11:20:23.593
Each machine MUST be upgraded to a Windows 10, if you don't want to pay for the upgrade, Windows 10 while then upgrade is being offered for free is not actually "free" and the offer ends on July 29th – Ramhound – 2016-06-15T11:41:24.030
From user Demz: Licensing is a Microsoft business, you can call activation licence support; maybe they can give you a alternative solution.
– fixer1234 – 2016-06-16T00:44:49.770