Hi Rakesh Gandhi,
Regarding your question, you can't in‑place convert a Windows Server 2016 evaluation edition that is already acting as a domain controller into a licensed Standard edition. Microsoft blocks that path because of the risk to Active Directory. That’s why your current DC in evaluation mode won’t accept a retail key. Since your current domain controller is running on the evaluation edition of Windows Server 2016, it’s important to note that Microsoft does not support converting an evaluation DC directly to a licensed Standard edition. Attempting an in‑place upgrade would put Active Directory at risk.
The recommended path is to deploy a new server with Windows Server 2016 Standard, join it to your existing domain, promote it as an additional domain controller, and then transfer the FSMO roles. Once replication is confirmed and your applications/data are migrated, you can safely demote and retire the evaluation server.
This approach ensures your domain remains intact and minimizes disruption. Please make sure you have a full backup before starting.
I hope you find something useful here. It's really appreciated of you to accept the answer as a way to share your experience with the community. Thank you :)
Vivian