Reinstalling windows 10 free upgrade, which key should i use

Hello my parents updated their laptop with Windows 7 to Windows 10, apparently by mistake. Since they do it themselves, I did not see the terms of conditions nor any information displayed before the upgrade. Since then they get used to the new appearance of windows so I did not do the rollback to windows 7 for them.

Now I start wondering if in the future I will do the disk format and fresh install of windows (which I usually do for them every 3 - 4 years), how will I be able to activate their copy of Windows 10 after fresh installation? Will I be able to use their Win7 key?

Please note that my parents do not have the hotmail account, and I don't want to force them to make one.

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