The free Windows 10 license is valid for one time activation only and tied-up to your old mobo and it's not transferable. You need to buy new license if you want to use Win 10 on your new mobo. Meanwhile for retail key, you can use it twice only on different machine. E.g. your current pc installed with Win 8, later you upgrade your proc, mobo, etc and you are allowed to activate again using the retail key. in future, you want to upgrade new spec again, you will not allowed to use the same key again. MS only allowed it to be activated 2 times on different machine.

Buying retail key means you can activate unlimited times on same machine. you can format/reinstall OS as many time as you like, activation will be successful. but not on multiple machine, even you only change mobo not whole new PC.

I'm sorry coz i couldn't provide you with the information since that is what understand last time during doing my job. I know the different between OEM and Retail. Just IINM, retail license would allowed you to activate twice on different machine with no question asked. If you need further transfer, you need to call MS.
Here is the thing I can suggest you. But you might not getting the Win 10 anymore since the free Win 10 in not transferable. At least you can try this.

What's weird though, from my last conversation that lady not even bother to ask about my Installation ID (9 sets of number). She just said I need to buy a new key.

But not that it matters anyway since, the only way I can retrieve my genuine Installation ID is by installing the old Motherboard back onto my PC again and running the slui.exe 4 command.

For future reference, here's what you should've done:

Upon upgrading, install BelArc Advisor and extract your Windows 10 product key. That way, when you reformat you can install Windows 10 directly and key in that product key to activate.

From their FAQ, it seems I am still entitled to transfer my retail key from my old Motherboard.

I have to say their customer service has become pretty bad since the last time I called them over a year ago for the same problem. Keep insisting with stock questions, and I can't help to think that the person behind the phone was taking me for a pirate.

From the FAQ above it is clearly written my Windows 8 serial key carries the rights of a retail version of Windows 10, and it is within my rights to transfer my key to another machine. Actually i'm not new in this situation, I have transferred my key twice before for Windows 8.1 via phone, not sure why I cannot do that anymore this time.

Moreover after reading the FAQ thoroughly, It seems there are no parity between Windows 7, 8 and 8.1 to Windows 10 serial key, as long as it is the retail version. So frustrating since I paid full price for my Windows 8 three years ago.

Are you sure? I've done this on the 3 PCs in my house and each one has a separate key.

Yup, this is indeed a mess. Even their own Customer Service Officer also doesn't know anybody who obtain a copy of Windows 10 through upgrade from retail Windows 7, 8 and 8.1 is entitled to the same rights as someone who owns a retail copy of Windows 10.

Disappointing because they not even bother to check my Serial Key and Installation ID. Trying to tell them my full story was a waste of time since they keep repeating and insisting I have to buy a new key.

@TS
Sorry TS tumpang your thread,as my question is quite similar.

Guys, i've just changed a new motherboard,since im using OEM Windows 10(Upgraded from Windows 8.1)how do i re-activate my license on this new board?

thank you in advance.

Mean i have to spend another few hundreds for new OEM copy? Oh gawd

Update:

I have took the advice from the service desk of microsoft to revert back to windows 8.1 and upgrade to windows 10 again,it works, my new mobo is activated already.

But they said,after the free program ended and you have changed new mobo again you will no longer able to get free upgrade to windows10. Either purchase new license for windows 10 or stick with current windows that comes with the seriel key.