I was thinking of you when I wrote that part WR. ;)
Microsoft has a top secret formula where Windows keeps track of hardware changes, & once it hits some threshold it will deactivate. The only thing I've ever read about it that's supposed to be for sure, is that a new MAC address on the motherboard's network chip will trigger deactivation. When that happens you get the notice on the desktop that this version of Windows is not genuine, Windows turns on some limitations, & you have to reactivate, usually within 3 days.
Now, will Microsoft lower that threshold? And will they have a [much?] more restrictive policy on getting Windows reactivated? We bought a family pack of win7 HP, which came with 1 key we can use on 3 PCs/Laptops -- we've had to call after every hardware change to any of those 3 PCs, but Microsoft has always given us a code to reactivate. If they stick with the same policies on the win10 upgrade we'll probably be fine.
OTOH Microsoft has a pretty restrictive license with Ofc 2013 - http://blog.chron.com/techblog/2013/02/sorry-you-cant-transfer-office-2013-to-your-new-pc/
If the free win10 upgrade carries a similar OEM-type license, it's likely the 1st upgrade you did would need a new license to go with it.
I may well be worried about nothing, & if Microsoft doesn't have any other special offers, might as well go with the free version & cross that bridge if/when you come to it. If they do have some other offers however, I'm thinking about buying a few licenses to have available as spares. I'd rather take a chance on wasting ~$100 for licenses I'll not use, rather than taking the chance I'd have to pony up $300+ later.
There's also the upgrade matrix that Microsoft's released. If you have the very popular win7 HP [Home Premium], you'll get win10 Home rather than pro. win8.1 pro = win10 pro. It's not entirely clear what the differences are yet, but it could turn out that many folks with win7 HP would be better off buying win10 pro, depending on special offers.