There are two approaches:
1st is to introduce new 2012 member server in existing DFS farm as new namespace server, add new name space servers to all folder targets, add replica of existing folders and after initial sync and replication completed, remove 2008 R2 server from DFS farm completely
2nd approach is I think what you wanted to do ?
Not sure if you can upgrade 2008 R2 server to 2012 with DFS replica ?
Best option is to remove 2008 R2 server from DFS replica, remove from name space servers and DFS, upgrade it to 2012 server, add it as DFS name space server for all folder targets, create DFS replica and allow for initial sync and replication
1st is to introduce new 2012 member server in existing DFS farm as new namespace server, add new name space servers to all folder targets, add replica of existing folders and after initial sync and replication completed, remove 2008 R2 server from DFS farm completely
2nd approach is I think what you wanted to do ?
Not sure if you can upgrade 2008 R2 server to 2012 with DFS replica ?
Best option is to remove 2008 R2 server from DFS replica, remove from name space servers and DFS, upgrade it to 2012 server, add it as DFS name space server for all folder targets, create DFS replica and allow for initial sync and replication
http://social.technet.microsoft.com/Forums/en-US/2f8bb88a-d6df-44ed-b6b0-6f75920bea67/move-dfs-from-windows-server-2008-r2-to-windows-server-2012?forum=winserverfiles
Mahesh