I can't prove or disprove this error condition, but here's what I think happened. All of my Dell Laptops were activated with KMS 2.5.1 and all of them were upgraded from either Windows 7 or 8 Professional with the 2.5.1 activations which worked fine. When I removed the Windows.old folder from these Dell's because I had no intention of going backwards, it appears that the Windows activation failed and I got the message to Activate Windows. What I did was uninstalled the KMS 2.5.1 and re-installed the KMS 2.6.1 and installed it and ran ez-activator. All of the Dell laptops came back with a current activation and a new Product Key. Again I don't know if the cause was because I deleted Windows.old
I deleted Windows_old using Windows cleanup and I am still activated. Perhaps this happened because of manual deletion of the Windows_old folder..?
If you activated with KMS that means they were a Volume License version, the free upgrade is not for any Volume License or Enterprise. Only retail non VL or OEM, products can upgrade for free.
I cannot confirm this: I have an old Windows 7 OEM key activated notebook - upgraded without problems to Windows 10 and automatically activated. After activation slmgr -dlv shows: "Retail" channel. I have a Windows 8.1 (legal) KMS key activated notebook - upgraded without problems to Windows 10 and automatically activated. After activation slmgr -dlv shows: "MAK" channel.