Chances are, if you have an environment with a large number of servers running Windows Server 2003, you have domain controllers running Windows Server 2003. If you have domain controllers running Windows Server 2003, your domain and forest are likely to be at the Windows Server 2003 functional level.
One of the advantages that moving to Windows Server 2012 R2 domain controllers will give you is that it will allow you to upgrade to the Windows Server 2012 R2 domain and forest functional level. These functional levels include the benefits available in all previous functional levels including (but not limited to) the following:
- DFS replication for SYSVOL
- Fine-grained password policies
- Personal virtual desktops
- Automatic SPN management for services running under the context of a managed service account
- Protected Users
- Authentication Policies
- Authentication Policy Silos
- Active Directory Recycle Bin
The drawback of upgrading the functional level of a domain is that it limits you to only running domain controllers running the operating system of the functional level or higher. For example, if you upgrade the domain functional level to Windows Server 2012, you can only use domain controllers running the Windows Server 2012 or Windows Server 2012 R2 functional levels.
Raising the domain functional level doesn’t mean you can’t have member servers running earlier versions of the OS. For example, you could still have member servers running Windows Server 2003 in a domain running at the Windows Server 2012 R2 functional level.
You can find out more about features available at different functional levels by consulting the following TechNet article: http://technet.microsoft.com/en-us/library/understanding-active-directory-functional-levels%28v=ws.10%29.aspx