A task IT admins don't have to do often is upgrade their KMS capabilities. The last time I did this MS was still using their volume activation portal, but as things moved to our O365 tenant I ran into lots of roadblocks and a lack of documentation to help. The biggest hurtle was locating the KMS key. Turns out I needed to call MS to get it. The rest was easy:
Get KMS key for new OS. (This is not the generic activation key for the new OS. This is our specific license key for activations)
Key may be available in MS license portal. If not you have to call MS support to get access to the key.
Volume License Key Phone Numbers | Microsoft Volume Licensing | Current # as of 12/2023 - (888) 352 7140
You will need to have your current agreement or enterprise number
If it is a VM always snapshot the server prior to making any changes to ensure easy rollback if something goes wrong.
Open admin CMD or PS and run the following commands
Uninstall the existing KMS host key: slmgr -upk
Install new Generic Volume License Key (GVLK): slmgr /ipk new-license-key-example
Restart software license service or reboot