A: If you have already installed Windows Server 2012 Standard, it's actually very easy to move to Datacenter provided you have a Datacenter key. At a command prompt, type the following and click Y when prompted (the machine will actually reboot twice, and after rebooting will be Datacenter). This process also works on Windows 2008 R2:
dism /online /set-edition:ServerDatacenter /productkey:<Datacenter key, e.g. AAAAA-BBBBB-CCCCC-DDDDD-EEEEE> /AcceptEula


Note: You can't just type in a Datacenter key in the Windows Activation control panel dialog box to upgrade--you will receive an error.

You can always check your current version, and the versions you can upgrade to, by using DISM as shown below:

dism /online /get-currentedition
dism /online /get-targeteditions

This server can't be a domain controller (DC) and would need to be demoted prior to perform the version upgrade using DISM. See also, "Q. Can I upgrade to Windows Server 2012 Standard from Windows Server 2008 Datacenter or Windows Server 2008 R2 Datacenter?"