Upgrade planning will involve licensing, so it's important to review the licensing details for your desired edition...
and budget accordingly. Generally speaking, Microsoft uses a per-processor licensing scheme for Windows Server 2012 R2.
One Datacenter or Standard edition license covers up to two physical processors per server. The main difference is the number of virtual machines (VMs), where the Standard edition license only covers up to one VM per processor and the Datacenter edition supports unlimited VMs.
Consider some simple examples. A server with two physical processors would need one Datacenter or Standard license. Suppose that server had three VMs; it's still one Datacenter license, but two Standard licenses. If that server had 12 VMs, it would still be one Datacenter license and six Standard licenses. Now a server with four physical processors would need two Datacenter or Standard licenses, but if the server had 12 VMs, it would take two Datacenter licenses or six Standard licenses. While the $6,155 MSRP price tag for a Datacenter license and client access licenses (CALs) far exceeds the $882 MSRP price for a Standard license, you can see how the Datacenter license can become cost-effective for highly virtualized environments.
It's always a good idea to discuss licensing calculations and pricing (especially volume discounts) with your organization's VAR or other channel provider and get a firm pricing commitment before embarking on a data center upgrade initiative.
The move to Windows Server 2012 R2 is often uneventful for skilled IT professionals, but any change to the production environment of a busy enterprise can result in unexpected and costly disruptions. IT employees should approach any OS upgrade by familiarizing themselves with the processes and testing the results in a lab setting in order to identify and resolve unexpected upgrade problems in hardware support or software compatibility.
Distilling the installation requirements for Windows Server 2012 R2
Find out the upgrade limitations for the latest version of Windows Server
Prepare for the upgrade to Windows Server 2012 R2