When determining whether to decommission or upgrade your Server 2003 Domain Controllers, the question you need to answer is whether you need on-premises central sign on.
When considering a migration from Server 2003 to Server 2012 R2, organizations should determine whether or not they still need on-premises domain controllers.
The most common role for a server running the Windows Server 2003 operating system is file server. Should you not be interested in upgrading your file servers to Windows Server 2012 R2, you could consider the possibility of migrating your file servers to Samba on Linux.
For some organizations that use only a small subset of Windows Server 2003’s features and who have appropriately trained systems administrators, it may make sense to migrate some servers to Linux rather than to a newer version of Windows Server
For most people, migrating away from Server 2003 means migrating to a newer version of Windows Server. Rather than upgrading to Server 2012 R2, some organizations might consider looking into alternatives such as another OS or hardware appliances.