I've done some experimentation with this and found an easy way to reactivate a newly installed VM.
A tip from tutorial Hyper-V - Optimizing Virtual Machines
A hard disk on a physical machine, or VHD on a virtual machine on an activated Windows 10 system can be replaced without losing the machine's activation status (digital license).
What I always do when I have activated a Windows 10 virtual machine is to remove its VHD, and export it without any disk. I now have an activated Windows 10 VM with no disk. When I want, I can just import the VM, attach a new VHD to it, and install the same edition of Windows 10 it had when activated. New installation will be automatically activated, based on the existing digital license.
I even have have a few exported virtual machines on a safe storage which have digital licenses for Home, Pro, Education and Enterprise editions; when imported back to Hyper-V, I can install any of the four editions, all of them automatically activated, or install all four editions on VM in multi boot, all activated.
Kari