Hey there. I run Linux on my daily but also work in a Windows-centric PC repair shop.
“Official” answer: You can move your key over to a new mobo by signing in to Windows with a Microsoft account, installing your new hardware, and activating Win 11 through the Settings->Activation->Troubleshooting (button)->“I recently changed hardware”. And that will pull your key back down from your account. But it does lock you into an account.
“Unofficial” answer: you can absolutely update to Win 11 on old hardware. The easiest way is to boot a Win 11 iso in Ventoy. That works fairly often. You can alternatively edit the installer to not do the TPM check in the installer, which you can search for guides for online/YouTube.
Alternatively: you can hop on g2a, kinguin, etc and buy Windows keys cheap.
To be clear I know this is all bullshit, but it’s options. Hope this helps!