In a short answer about computer operating systems.. "one copy, one install, one system, one user" (Administrator) is the rule. If the OS is used on another system, the OS may sense major differences (chipsets, processor type, users) and shut down.. permanently. Counterfeiting and piracy are major problems with almost all software. So you can't blame the provider for being quite a bit on the protective end of the deal.
All the issues are spelled out in the EULA, which you are encouraged to read and accept.
If you migrate the OS to another near-identical system, likely no major issues except minor drivers. But if major differences, the OS goes belly-up.
Regardless of OS version, the proper drivers for motherboard features are on the associated disk that comes with the board. If any revisions, they are made after the OS has been activated. Best to source drivers directly from the provider, not from "third-party" sources. This way, little chance of conflict. The Microsoft® site also has the latest sources, and they should be identical to the OEM source.. or mildly reconfigured to be 100% compatible to the OS version.
Yes, you can reactivate an older OS.. as long as it is still actively serviced (supported) by the provider. Windows XP is in an extended support period, not actively supported. Windows Vista also is not actively supported. The major proof you need that you are the sole user (lessee) of the OS and system, and that there are no other copies active. And that the COA is unique to the install.
As to a work-around, yes you can use the 'upgrade' OS install as a primary install.. as long as you provide proof of a previously acceptable OS version when requested. But you still may need some drivers and programming to be up to standards. An easy way is to do a basic install of the older OS, do the driver install, then do an upgrade before adding other programming, after the activation. You may need to update drivers for the newer OS version. Ensure they are digitally signed in accordance with the WHQL listings.
It may require a telephone call to Microsoft® or other OS provider, to certify that you are indeed the sole user of the OS and system, with the indication that "the hard drive crashed" or "the PSU fried my computer".
"The goat ate my software" is not acceptable!
May be a bit of a challenge, but don't let it deter you. Either go over it, under it, around it or through it! MS is willing to work with you and keep you as a happy client.
F@H.. to solve mankind's maladies.. in our lifetimes!