Using Win 11 volume license image/ISO/key on Win 10 devices while upgrade is free?
You can create a new Win 11 golden image, capture it to the SDA & re-deploy it to the client machines as an image refresh as it will bypass the Win 11 pre-requisite checks (TPM & CPU requirements).
However if you intend to not intervene, you might want to consider setting up Windows Update for Business for seamless in-place upgrade provided your endpoints complies with all the pre-checks.
Regarding the license key, according to this you may use a win 10 license on a win 11 image. The machines in my environment has an OEM bios windows 10 key and they are still able to be activated on windows 11.