Procedure for Activating Windows Server 2012/2012 R2 Evaluation version into Retail(Full) Version is not possible with the GUI wizard.
It can be done only using Command-Prompt.
Follow the below procedure for activating Windows Server 2012/2012 R2 Evaluation Version into Windows Server 2012/2012 R2 Retail(Full) Version-
i) Open Command-Prompt with Run as Administrator.
ii) Enter the following command to check the current edition of installed operating system-
dism /online /get-CurrentEdition
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iii) Enter the next command to upgrade it to full version (Standard Eval to Standard Full Version/ Datacenter Eval to Datacenter Full Version/ Standard Eval to Datacenter Full version)
Note:- You cannot downgrade the version. Ex- If Current Edition is Datacenter Eval edition, then it cannot be downgraded to Standard Full version.
Run this command to upgrade it to the full edition-
dism /online /Set-Edition: ServerStandard /ProductKey:"Enter your Product key here" /AcceptEula
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iv) Enter "Y" to restart the computer.
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v) Click Close.
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It can be done only using Command-Prompt.
Follow the below procedure for activating Windows Server 2012/2012 R2 Evaluation Version into Windows Server 2012/2012 R2 Retail(Full) Version-
i) Open Command-Prompt with Run as Administrator.
ii) Enter the following command to check the current edition of installed operating system-
dism /online /get-CurrentEdition

iii) Enter the next command to upgrade it to full version (Standard Eval to Standard Full Version/ Datacenter Eval to Datacenter Full Version/ Standard Eval to Datacenter Full version)
Note:- You cannot downgrade the version. Ex- If Current Edition is Datacenter Eval edition, then it cannot be downgraded to Standard Full version.
Run this command to upgrade it to the full edition-
dism /online /Set-Edition: ServerStandard /ProductKey:"Enter your Product key here" /AcceptEula
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iv) Enter "Y" to restart the computer.
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v) Click Close.
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Hope it helps!
If any issues, please post your comments/feedback so that we can provide much better solutions.