Well for one I'd look at the issue of whether the terms in the EULAs are actually *legal* or not.
The answer is 100% CERTAINLY that many EULAs terms (including Microsoft's specifically) are manifestly illegal in certain states and entire countries. In many places in the EU, for instance, you're GUARANTEED by law certain warranties of fitness for a particular purpose, et. al., and several kinds of exchange / replacement / refund rights.
Even in the US here there's ample case law (e.g. the Doctorine of first sale) that would invalidate certain purported EULA restrictions from being in compliance with Federal law, and several courts have upheld that.
Also there's nothing invalid about replacing a mainboard even for OEM software, it's allowed to sell it, and it's allowed to replace a problematic one with a new one as well, as you'd surely expect to be the case.
e.g. if you got a new Dell PC with Vista OEM preinstalled, your motherboard's USB port never works due to a motherboard defect so they swap out your motherboard under warranty you certainly wouldn't expect that they'd be saying "well now the motherboard you paid for actually works, but, uh, sorry, now you need to buy a new OS, bad luck there, will that be cash or charge?".
The STANDARD response to REPAIR virtually all computer problems at most computer repair services like, for instance, Geek Zone, et. al. is to replace the most likely problematic component and reinstall the software from scratch to correct any problem. Even if in most cases it's a software fault or driver fault or whatever, would you consider paying a technician $100 initial fee + $100 / hour + cost of parts to *diagnose* and/or *physically repair* a $75 new cost motherboard? Of course not. If it's not working well enough for the curstomer for whatever potential fault / reason, repairing it == replacing it, then you do a clean reinstall.
The only difference is if YOU buy the OEM software, YOU are the OEM, you're the one that has to "warranty" the computer to perform up to some level of reliability / performance / expectation. If there's any problem with it satisfying your expectations of reliability in a reasonable 'warranty' period (e.g. 1 year or whatever) it's perfectly normal and accepted industry practice to replace any part(s) of it as quickly as possible to get back to business with minimum cost / diagnostics / delay.
So if the OEM (i.e. you) have a problem with the function of your existing PC of course you have a right to replace any problematic parts of it with alternative ones so that the overall system functions stably and suitably for you. If it didn't do so if you bought it from DELL, HP, COMPAQ, you'd certainly return it for refund / repair. No difference if you're the OEM, repair it or get your money back for the problematic parts, etc. but there's NO reason you'd lose the right to use UNRELATED parts of the system just because it was necessary to do some replacements / alterations to other parts of the system for whatever stability / functionality / compatibility defects you've had.
So in most cases I don't see that there's any problem in saying one has had to repair the function of a system and incidental to doing so made X, Y, Z changes to the hardware. It is still your computer SYSTEM, and that is what Vista is licensed to, just because I replace / repair parts of it, it's still my system, and I'm still not using the software on any OTHER and additional system, which is really all MS probably has legally the right to be concerned about in most jurisdictions however much they may prefer to deceive customers into believing.