You misunderstand, going through the upgrade process converts the old Windows 7/8/8.1 license into a Windows 10 license. The offer is a time window to allow users to convert their licenses to Windows 10, not a trial period to use Windows 10. It will not matter once the offer is over because the user will already have a Windows 10 license. Remember, Microsoft wants everyone to move away from Windows 7/8/8.1, they won't force everyone to downgrade again for no reason.
See here.
Digital entitlement is Microsoft's term for their new online activation scheme that does not require a product key. As long as the computer has has Windows 10 activated before, a record is kept on Microsoft's servers that will automatically reactivate the same PC if Windows 10 is installed.
If you activated a free upgrade to Windows 10 or bought and activated Windows 10 from the Windows Store, you have a digital entitlement for your device. This means you can reinstall the same edition of Windows 10 that your device has a digital entitlement for without entering a product key. During reinstallation, if youre asked to enter a product key, select Skip. Windows 10 will automatically activate online after the installation is complete.
It says at the bottom at "Activating Windows 10 after a hardware configuration change" that:
If you upgraded to Windows 10 using the free upgrade offer and then made significant hardware changes to your device, such as replacing the motherboard, Windows 10 might no longer be activated. For help on how to activate your device, contact customer support.
In the past, older versions of Windows that activated with license keys could also be somewhat tied to hardware. If you performed some parts changes/upgrades and tried entering the old key when reinstalling Windows, sometimes it would reject the key as it believed the computer it was being installed on was a new and different computer compared to the one it was originally activated on. In those cases, you would also have had to contact Microsoft support and get help manually activating Windows. In Windows 10, it is no different. All that's changed for digital entitlement is that you have no Windows 10 product key, just the old Windows 7/8/8.1 key (well hopefully you saved it somewhere).
They do it this way so people can't cheat the system and get Windows 10 activated on different PCs other than the one they have the old copy of Windows 7/8/8.1 on. They're not simply handing out free Windows 10 license keys and for new PCs without an OS. They want you you upgrade away from Windows 7/8/8.1, hence the media creation tool and the targeted upgrades for older Windows users.
Are you doing heavy overclocking? If not, simpler and smaller air coolers will be fine.