upgrade to Windows 10 for 2nd time from windows 8?

[jaystarter]

Started off with laptop windows 8 OEM pre-installed which over the course of time updated to Windows 8.1 and from there to the free Windows 10 and currently Windows 10 anniversary latest.

It is now beginning to have problems and slow down freeze etc so I would like to do a factory reset to the original state i.e. :(Windows 8 on a recovery partition + I made an image to usb + recovery start up disc as advised by Windows) to start again.

My question is would I be able to upgrade to the free windows 10 again? A web search shows the legit download is available but I don't have a windows key for the original Windows 8 as it was pre-installed and the updates to Windows 10 never required one .

[HansV]

Can't you use the built-in reset option of Windows 10? This should reinstall Windows 10, keeping your documents if you wish.

Start > Settings > Update & Security > Recovery > Reset this PC

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[viking33]

There are a number of utilities that can probably uncover the key. Try this site and check out a number of them.
Magic Jellybean and others

[jaystarter]

Looks like my thinking was over complicating the issue. Easiest way looks like Extract the product key as a safeguard and let Windows Recovery and Reset do its thing.