Upgrade your Server 2012 Standard R2 edition to Datacentre

Firstly run the following command to get your current edition:
DISM /Online /Get-CurrentEdition

You should also verify what media type you are using for the current OS.

And then run the following to view available upgrade paths:
DISM /Online /Get-TargetEditions

And finally to perform the upgrade run the following (where X = your product key for datacentre:
DISM /Online /Set-Edition:ServerDataCenter /ProductKey:XXXXXXXXXXXXXX /AcceptEula