Activate Your Evaluation Version
If you install Windows Server 2012 Evaluation Edition and later purchase a license, the activation process is a little more confusing than you would immediately think. It's a little more work than simply typing in a new product key. First, you must convert to a full retail version. To see what version you are currently running, type the following in CMD or PowerShell:
This will show you a Window with full details on your version, build and activation information. The most important line is this:
Name: Windows(R), ServerStandardEval edition
This lets you know you are running an evaluation version, which can be converted to Standard or Datacenter editions (abbreviated as ServerStandard and ServerDatacenter, respectively).
To convert your version to retail and activate it in one go, run the following command from command prompt or PowerShell with elevated (Administrator) privileges:
DISM /online /Set-Edition:ServerStandard /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX /AcceptEula
Replace ServerStandard with your edition if you have ServerDatacenter or ServerEssentials. Replace XXXXX-XXXXX-XXXXX-XXXXX-XXXXX with your product key.
When you try to upgrade Windows Server 2019 Evaluation version to a licensed full version using a MAK volume license key you may receive below error message:
The specified product key could not be validated.
Check that the specified product key is valid and that it matches the target edition.
To fix this first you have to install a KMS client key using DISM command line tool.
Keys are available on https://docs.microsoft.com/en-us/windows-server/get-started/kmsclientkeys
(Get your respective key from “Windows Server LTSC/LTSB versions” section)
Now open CMD or PowerShell with administrative privileges and execute below command.
DISM /Online /Set-Edition:ServerStandard /ProductKey:N69G4-B89J2-4G8F4-WWYCC-J464C /AcceptEula
Once the above command is successfully executed you will prompt for restart the server. Restart the server and execute the below command to install the new MAK key which is available on Volume Licensing Service Center.