My wife's Win 10 laptop (10 year old Thinkpad, was originally XP) was slowing down to the point she was constantly cursing it - and by implication me for not fixing it. It was fine when it originally upgraded itself from Win 7.

One of my options was a clean reinstall of Windows 10 from the download provided by Microsoft on the internet. That is definitely possible - I know of people who've done it recently. At the same time I could change to a Solid State Disk (SSD) - about £/$60 for 500GB which would enliven this old machine no end. You can get smaller cheaper ones.

My main concern, if I changed the hardware, was that MS would not register the updated system. So I converted my local login to a MS account because I read that might help.

I did install the SSD. It was very easy, and it goes very much faster - it starts up in 15 seconds. So I recommend that if you have an old laptop. I also bought a SATA/USB3 cable so I could plug the old disk in to recover the data. (You can also clone the old disk if you don't want to reinstall.)

I didn't reinstall Windows 10, I installed Linux - so that's off-topic. But I can go back to Windows if my wife can't live with the new setup by just replacing the disk.

The only thing I miss about Windows 10 is File History backup, which we did to a USB drive. If you don't use that, take a look at it. But don't keep your drive plugged in all the time, as Win 10 wants, to avoid ransomeware getting at it.