I think @JaredBusch is right from what this page says:
https://www.microsoft.com/en-us/software-download/windows10
Note
If you’ve already successful activated Windows 10 on this PC, including if you upgraded by taking advantage of the free upgrade offer you won't need to enter a Windows 10 product key. You can skip the product key page by selecting the Skip button. Your PC will automatically activate later.
If I had to take a guess from previous versions, if one of three main components changes it will go ahead, and anything else will requires a phone activation or something. I have a Windows 7 machine with 32 bit windows on it and decided to test this theory. That was 18 hours ago and it's at 99% now... To be fair, this is on a 2.2GHz Core 2 Duo.