MS has not ever to my knowledge ever enforced this particular restriction of OEM keys. The restriction is there so systems integrators can't sell a PC with the same copy of Windows more than once. They couldn't care less if an end-user buys an OEM copy from Newegg to save a few bucks and upgrades their PC, and furthermore there are more than 1 billion PCs with Windows installed worldwide, MS isn't going to expend pointless resources to try and police all of them like that.
I've bought OEM copies of Windows. I've overhauled my PC afterwards. You know what happens? Windows prompts for reactivation. You can typically reactivate instantly over the Internet 2 or 3 times. Then it will fail and say you have to call the activation hotline thing. So you do that, it makes you enter some numbers, then it gives you numbers back, you put them in the activation screen, and Windows activates.
MS has better things to do than try and police a billion copies of Windows.
No, they just don't care about corner cases. The reality is that you will be able to download an ISO of Windows 10 from MS to do a clean install. They really dragged their feet on making Win8/8.1 ISOs available though, so hopefully they will be quicker with Win10 since they are anticipating people will want to clean install upgrade from Win7/8.1 to 10 with this free offer.
MS wants people on Win10 because security and patching 4 different versions of Windows is a nightmare for MS. Period. There's no evil ulterior motive here. They had to support WinXP for more than a decade and even had to pull resources from the long-delayed project that became Vista because of all the security problems XP had. They don't want to ever support 4 versions of Windows again, so much so that they are willing to give Win10 away to every person who's using Win 7/8.1, EVEN PIRATES, just to get them off the old goddamned OSes and onto the latest one. MS isn't doing this because they love you. They're doing this because they love themselves too much to keep forcing themselves to support 4 goddamn versions of Windows forever.