If not, and we only get to migrate the activation key from the previous server’s hard drive to be used in the guest OS on the new hard drive, does that mean we have to pay for another activation key for the host OS on the new hard drive?
Not clear on your ask but I do have a couple video on doing migrations:
A VHD can be imported directly into XCP-NG, I’ve done it. But licensing often breaks on things that use the NIC or other serial number in the licensing process. Not sure about Windows (or other OS) activation.
I’ve also failed on several of these migrations, kind of a try and see. Fix what breaks, or maybe change plans. I’m guessing you are talking about a Windows Server install, does the original server still function? If it does, then you might be able to install Server Backup role and do a full metal backup. That should let you start a VM, boot from an install disk, select repair and use the backed up install to install it to the VM hard drive. It’s a long way around, but I believe this is designed to make it successful when you have a complete hardware failure.
Getting the backed up drive connected to XCP-NG was a challenge that I didn’t figure out, Tom has a new video on USB passthrough that might have helped me back then. Backing the server up to a file share might also work, and might work better with things like XCP-NG.
All of the above is a guess except the VHD import, but also only a qualified success with a single server that I moved. I should also mention that everything except one server is activated with KMS because we have an enterprise account.