This article provides information concerning the conversion of Windows source machines with OEM licenses.
After performing a conversion of a Windows source machine that has been activated with an OEM license, the target virtual machine prompts for re-activation when booting into Windows for the first time.
The reason for this is that when performing a conversion, the OEM license is being transferred from one machine (source machine) to another (target virtual/physical). However, as per the following Microsoft article - an OEM software license may not be transferred from an old machine to a new machine-even if that machine is no longer in use. The OEM license is tied to the machine on which it was originally installed and can't be transferred to other machines.
This information was taken from the following Microsoft article:
For customers who have experienced this issue, the recommended workaround is to perform a Volume License upgrade or similar. Otherwise, it is advised that the user ensure that the source machine has a valid retail or Volume license installed on their source machine before attempting the conversion.