Here is what I have found so far playing with the upgrading.
1) PC with Insider build cant upgrade to Pro retail & keep settings, decided to do a clean install & see what it does, started with Windows 7 Pro & thought I would test the daz loader & see it it upgrades. Did the upgrade & it worked like a charm, will later do a clean install & see if it works again.
2) PC 2 had Windows 7 Home Premium on did the upgrade to 10 Home & decided to see what the product key is before doing a clean install Aida64 picked the key up, but i decided to make sure I get the same result & googled a bit & found this:
https://www.herdprotect.com/showkeyplus.exe-daa1d0c3c45be1652c149fbbe5824137aeb7ac82.aspx &
http://www.tenforums.com/software-apps/2577-showkey.html
ShowKeyPlus listed my original 7 Home key & the new 10 Home key, Aide & Showkeyplus, gave the same producktey. Did a clean install without enetering a product key, activated once done & checked the keys again. This time Showkeyplus only dislpayed the Win10 key.(made sense as hdd was wiped, but still same keys as with upgrade)
3) PC3 Also had Home Premium, I did the upgrade, & again checked the key with Showkeyplus but decided halfway trough the upgrade to enter the product key during the install & see if it activates. Entered the key on the last request (before windows logs in) & it accepted the key, Activated & it worked. I added more memory on the 3rd machine & it worked without activation.
I will be doing a test at work Monday & see if I change the mobo on a machine if it will ask for reactivation, just to see what happens.
Its still unclear how the licensing will work ie if you upgrade your machine & change the board you would have to purchase windows, & how the original product key ties in with your current key. It definitely needs to make a connection with your key & current hardware to be able to do a clean install without entering a key & then getting the same key. Cant think that the media could have the product key as I used the same Pro to install 4 machines already & the Home version on 2 machines.
I am saving my current Windows 10 Keys just for in case.