Upgrade to W10 with official W7 key but non official W8.1 installed

Current situation: PC has an OEM key from Asus for an W7 home edition. PC at this moment is running W8.1 pro not licensed.

Goal: Update the PC to W10 without losing any information. That also includes the registery for custom installed software and programs.

Tried 1: Run the windows 10 usb media tool to upgrade from the unlicensed 8.1 and entered the W7 OEM serial. Not working. Giving a message that the serial is not valid.

Tried 2: Made a backup of the running W8.1 unlicensed software and then downgraded to windows 7. Stored info to the folder Windows.old. But not all software and registery entries were saved.

Question: How can I upgrade a non licensed W8.1 pro to windows 10 with a legit W7 home key without losing data?? So no clean install. A real update.

Thanx

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