Depending on the complexity of your DFS deployment, a migration from Server 2003 to Server 2012 R2 can be straightforward or frighteningly complex.
If you are keeping the same environment, you can add new servers running Windows Server 2012 R2 to host each DFS namespace, while retiring existing servers. In a similar manner, you can add new servers running Windows Server 2012 R2 to host new copies of existing DFS replicas. Once replication has occurred, you can then gracefully remove the existing replicas.
In the event that you are migrating to a new forest, you can use a utility called DFSUTIL to export a specific namespace to an XML text file. You then use DFSUTIL to import the DFS namespace in a new domain. You can also use the ability to pre-seed DFS replica content to migrate content from the DFS replicas in the existing forest to the new DFS replicas in your new Server 2012 R2 forest.
You should start your reading about migrating DFS namespaces by consulting the following blog post by the Directory Services team (it deals with 2008, but the advice applies to Server 2012) http://blogs.technet.com/b/askds/archive/2008/01/15/migrating-your-dfs-namespaces-in-three-sorta-easy-steps.aspx
The Windows Server Migration Tools also include tools for migrating DFS Namespaces and data. However the Migration Tools are designed for migrating data sets of less than 100 GB of data. If your DFS deployment hosts more than 100 GB of data, you may need to use tools such as RoboCopy or backup and restore utilities. You can find out more about using the Migration Tools to migrate DFS at the following address: https://technet.microsoft.com/en-us/library/jj863566.aspx