Yeah I totally ran into that issue. All it takes is a call to Microsoft in India and let them reset your activation key..... They'll do it. Just tell them you changed your motherboard and they'll do it. They did it for me
M$ chose to skip Windows 9. Why? Some think it's because 9 is unlucky number in Japan. Other's say it's because M$ already used 9 (Windows 98). Probably just because 10 sounds better.
Just like Apple skipped iPhone 9 and went to X.
Windows 10 is actually version 6.4 and Windows 8.1 is version 6.3.