As I understand it, they only look at the motherboard for Windows 10 licencing.
The way Microsoft determines the hardware ID is tied to the mechanism they use to deactivate Windows after a hardware change, and the exact parameters are a very closely held secret. The VM community has figured out that one of those parameters that's given extra weight is the MAC address. The hard drive itself has 3 IDs, one hard coded, one that shows up with the command prompt dir command, & one that's used by their BCD boot setup to identify the drive partition to boot from. In my own experience with VMs & such the 2nd of those IDs can have an effect, but obviously so can the 1st -- you can set the 2nd with a utility from SysInternals at microsoft.com.
Microsoft uses the license key you purchase, or that was inserted via volume licensing, to verify that the copy of Windows you're installing is legit, but they also record the hardware ID to tell whether it was already used on other hardware. Their secret formula tells them how much hardware has changed, & if it exceeds whatever threshold, it's considered another device entirely, & you have to reactivate. With 10 being a free upgrade to many, they've set it up so that there's no need for a key, & more importantly for Microsoft, no need to create millions & manage more millions of keys.
Rather than let you use the win7-8.1 key you already have with a fresh install, it's easier & simpler for them to require an in-place upgrade from a activated Windows installation. At that point they associate a valid license with the hardware ID, same as always. And as I found out with my Windows tablet, they also store hardware data with the Microsoft acct. if/when one is associated with that Windows install.
As far as the motherboard goes, that might be one of the hardware IDs that goes towards the overall hardware ID of a device or system, but it is not the only one. And changing it may or may not trigger deactivation. I installed 8, upgraded it to 8.1, & never really cared for it, so it just sat there, I'm a bit ashamed to say for years without being fired up once. I even wiped it off the hard drive to use that partition for the win10 Insider Preview last fall. Late last year I replaced the motherboard, CPU, hard drive, power supply, & RAM. Then in early August I restored a image backup of 8.1, figuring I'd have to reactivate it before I could do the 10 upgrade. I was wrong -- 8.1 was still activated. 8.1 was also tied to a Microsoft acct., which might have had something to do with it.
Replacing the disk drive had no effect on the activation.
Feeling more of an optimist this morning, THAT is very good to hear - much Thanks. As I mentioned [somewhere] I've read a lot of reports that other people couldn't duplicate -- too many for them all to be mistaken or lies. I take your report plus all that info as meaning that Microsoft is being more flexible, tweaking their hardware threshold a bit to hopefully arrive at something more workable.
That said, I suppose I should also come down to Earth long enough to recognize that Microsoft has for years been a bit flaky with Windows activation, deactivation, & reactivation. Win7, & especially win7 HP, & especially, especially win7 HP sold as a family pack, was notorious for being hit or miss regarding whether you had to call for activation [&/or reactivation] or not. 8-8.1 might have been the same way, but I never paid as much attention to those versions so I don't really know.