Just buy licenses once on each machine and Windows figures it out from then on. I guess it phones home when it sees a hardware change and the server says "yep that serial number is licensed".

At least, that's how it worked for me in Palo Alto. For some reason with the new setup Windows complains about activation after every reimage, until I click "troubleshoot activation", and then it phones home and figures it out? I need to debug this.