W-10: Upgrade CPU - Does it invalidate product key registration?

Should be fine, the major issue in Microsoft "Seeing" a change of device is the changing of the motherboard. Even here if you log into windows with a Microsoft account, (before you change anything), this ties the record on Microsoft's server to the account, and this will allow you to recover a key even after a motherboard change.

To be honest, Microsoft are usually quite understanding if you are put through to a representative and explain what you have done and why, the Account method automates the system without the need to speak to anyone.

Obviously you will only be allowed to change a motherboard occasionally, Microsoft would not allow this to be happening many times

jrw11 said:

I did the free upgrade from W-7 to W-10 via a clean install where I used my Lenovo's MS product key. As I understand it when this was done MS records certain hardware information along with the product key so that it cannot be used on another PC...

Welcome to TenForums @jrw11

Almost, but not quite.

As I understand it the upgrade to Windows 10 would have got you a Digital Licence linked to the unique hardware ID of your PC and stored on Microsoft's activation servers. There is nothing in this digital licence to link it to the product key you used to activate W10. Your Windows 10 will have been given a generic product key.

Quite separately the key will have been recorded as having been used, but that just records its use, not the PC on which it was used.

Others have reported that just upgrading the CPU hasn't lost their activation. If you have Pro installed, you can use the activation troubleshooter if you have any problems.

Use Activation Troubleshooter in Windows 10

As Barman58 says, if all else fails MS are generally very helpful once you explain the situation to a representative.