After the upgrade, it is possible to activate the upgraded OS using your regular KMS server or MAK key. Using the same trick, you can also change the edition from Enterprise to Windows 10 Pro if you need. When you purchase directly from Tech Smith, your email receipt will contain your software key.You cannot change the edition with DISM commands or using any other method.
After Windows Server 2016 was announced GA on Ignite 2016 I installed the 180 day evaluation version of the Standard edition. Checking winver.exe: In an elevated prompt type the following: slmgr /ipk WC2BQ-8NRM3-FDDYY-2BFGV-KHKQY Again, this is the KMS key for Windows Server 2016 Standard Edition.
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Trial edition is always like Enterprise edition and is called as Enterprise evaluation edition and yes you can upgrade to licensed version by doing inplace upgarde as per support matrix Do you mean upgrade from Evaluation to the full Enterprise? Take a look at the link here on BOL to show supported edition upgrades (scroll towards the bottom, it has SQL Server 2014 upgrades between editions and what's possible).
I am adding this answer as it could be helpful and related to Inplace upgarde First you need to check using the link Thomas gave whether inplace upgarde you are doing is supported or not.