Windows activation after bios update

Exactly! Microsoft won't do anything about it and may never. Windows sees it as a new MOBO on many devices. This has been a problem for a long time (2018) and has not been fixed by anyone. Wonder why manufacturers don't fix this as well as Microsoft??????? Something else is under the wood pile and it is a giant snake.

The CPU may not be considered a hardware change.

People with the troublesome models need to be very careful with BIOS updates. My factory OEM machines seem to not have this issue so far. But I have stopped updating all BIOS's. So, it is difficult to say if those units are now affected.

I do not want to buy another copy of Windows that I don't need.

OEM built machines under warranty may have recourse (1 year?).

Long term I will give away my older units or use parts to build a new one and buy a new license. Or stop building units forever (fun while it lasted)!

Except that a BIOS update is not a hardware change is it?

Well said. If this hadn't worked I I had an ISO of linux Mint waiting to step up to the plate. Been thinking of doing that for long time but was held back by printer drivers. Finally managed to sort that too. So if I am "forced" to fork out again, the bye bye MS, simples.

I read somewhere (can't find it now) that CPU is no longer used/TBD

I was under the impression that MS linked the CPU id to the license? The BIOS serial shouldn't change from a simple firmware update!

Activation helps verify that your copy of Windows is genuine and hasn’t been used on more devices than the Microsoft Software License Terms allow.

When installing Windows 11, the digital license associates itself with your device's hardware. If you make significant hardware changes on your device, such as replacing your motherboard, Windows will no longer find a license that matches your device, and you’ll need to reactivate Windows to get it up and running.  

To activate Windows, you'll need either a digital license or a product key. To find out which you need, see "What activation method do I use, product key or digital license?" in Activate Windows. Then, use the following info to help you successfully prepare for a hardware change and reactivate Windows.

Reactivating Windows after a hardware change - Microsoft Support

The workaround was used by Microsoft tech support too, even if was an unofficial script, as reported in an article I linked a few pages before.

The curious thing about your post is you want to be certain about a workaround, and still you're using an OS which give you 0% control of what it does under the hood. The script simply do what tech support can't (or wont): give you the right to use a software you paid for, broken after a legitimate upgrade of the BIOS.

I gave up after 4 ticket opened in a month, all of them closed and flagged as 'resolved' when they did solve nothing, they never answered, and they still look to the other side leaving this well known issue to be. So I used the script as many did, and I'll use it again if a future BIOS update trigger some issue

Simple as that...