Thanks for the clarification!
How to Upgrade to Windows 10 - Microsoft
Availability was July 28 or 29, 2015 - I don't remember which.To take advantage of this free offer, you must upgrade to Windows 10 within one year of availability.
If you do the free upgrade that PC gets a digital entitlement. Go to Settings > Update & security > Activation > Product key. Mine says "Windows 10 on this device is activated with a digital entitlement." If I check my product code with showkey it's listed as VK7JG-NPHTM-C97JM-9MPGT-3V66T. All 4 of my PC's that upgraded to 10 Pro have that same key. So do all the other PC's that upgraded to 10 Pro. The one key per PC is history with digital entitlements. Also, if I clean install I can click skip or enter that generic key and it will activate with that digital entitlement. My original Windows 8.1 Pro Retail key is no longer tied to this PC. I can use it on another PC if I want. I have the same rights of use as I had before I did the free upgrade. Yes, if I use in quick succession on several PC's activation will likely fail, but that's how it's always been. As long as it's only in use on one PC at a time I should be legal.