Clean Install compared to "Keep Nothing" option.

I'm doing this with what I consider to be a healthy bit of paranoia, clean upgrade then wipe and reinstall using the same Win 10 image used for the upgrade.  There have been a couple of machines I've done this with that needed drivers that weren't covered by Win 10's generic ones the reinstall picked up but pretty much everything else worked fine and the hardware upgraded once I got online (Wifi &/or NIC driver checking is a must for older machines).  So my view is if these are in small numbers yes clean reinstall afterwards check everything connects.  In practical, objective terms though probably no big advantage if you're not keeping anything other than you need to dump windows.old at some point.

Just my $0.02 (now approx €0.02!)

I tried the clean install + win7 key method and it would not work (it did not accept the key).
I had used jellybean and some other tool to verify my win7 key.
I checked my key was genuine on the miicrosoft ws

I spent 20 minutes on the phone to Microsoft and they said that the above method does not work.
I insisted is should .....
 .... they checked again and reiterated that the above method does not work.
The upgrade/keep nothing worked flawlessly.

Were you using the latest .iso of Win 10?  Which version (ProductID) did your keychecker get verified as?
Generic OEM keys from royalty manufacturers, whilst valid, can't be used to activate but are the codes that keycheckers retrieve.

You'd have had to use the key on your product key sticker on the PC, not the one jellybeans finds. Also, you must install the matching version of Windows 10. If your Windows 7 was home premium, you can't install Windows 10 Pro.

It was .....
Product Name: Windows 7 Ultimate
Product ID: 00426-OEM-8992662-00006
Installed Key: FJGCP-4DFJD-GJY49-*****-*****

There was no sticker on the PC
I used the very latest ISO

I didn't know about the jellybean/sticker issue.

It doesn't matter now - I won't be doing any more free upgrades.
There appears to be no compelling reason to do a clean install, so I'll probably leave things as they are.

That's an Acer OEM key that's been leaked into the wild

Interesting MASQ - thanks. So much for online validation!

 At least I'm genuinely licensed now with win8

Did you install Windows 8 or was that the original OS the PC came with?

If the PC originally had Windows 8 on it, and it was downgraded to Windows 7, then the 8 product key would be embedded in the BIOS. Such PC's can be clean installed with Windows 10 and the OS gets automatically registered and activated.

I bought it second hand about 15 months ago.
I came with Win7 (It never had w8)
I Never had any activation or updates issues.
Every validation tool / website indicated that I had a genuine ms key

Thanks to everyone.

You are very welcome