tech newb: Do I need to buy a Win 10 key?

Hi guys, so I bought a used PC and the user removed the HDD, which is fine. But after reading the details about Windows 10 PC it mentioned that:

Will not allow for a change to a different motherboard model once installed

...so if the person who had the PC before had Windows 10, is that information saved onto their motherboard so I don't need to buy Windows 10? Just pop in my own HDD and I'm good to go?
thanks for the help. :)