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.

1. Put back the Mobo B into your PC and check if the Win 8 copy is activated. Then follow below step:



2. Put back Mobo A (from RMA), then install Windows 8 again and activate it. If still failed, try below step:



If you successfully activate it, try to check if it still possible to get the free Win 10 upgrade. It might be your luck. Hope this will help you.