I know that there is a lot of information on the internet, but I can't find any good advice for my exact situation.

I have two computers:
1) my own custom built PC with an OEM Windows 8.1 Home upgraded to Windows 10 Home (free upgrade when it was available), I installed the OEM version myself from a disc given to me by a friend, whose PC building company had bought one too many.
2) second, older custom built PC with windows 7 Home full retail version, with a reserved upgrade to windows 10, not used yet.

What I want to do:
Put the motherboard and major components from the first into the second and throw those out from the second. Then, put new motherboard and major components in the first.
1 will keep the same SSD, 2 will get a new SSD.

So, I think I can upgrade my windows 7 retail licence on PC 2 to windows 10 using the reserved upgrade (apparently this maintains retail rights) then transfer it to my new PC.
Then I want to move the Windows 10 OEM licence with the motherboard, which I believe will be the easy bit because I also understand that windows is automatically registered to the hardware, so it will do this for me when I transfer this.

Any faults in my logic here?