A couple clarifying questions:
- How much data are you migrating (# of files and total size)?
- Do you plan on keeping the replication active following your migration, or is it a one-time transfer?
Assuming you really do want to use DFS migration, I would use a single replication group. This reduces the complexity, chance for problems, and the amount of size required for staging files.
That being said, if this is a one-time transfer and if these servers are on the same LAN, I would run this command from your new Windows 2008 server instead of using DFS for this:
robocopy "\\oldserver\share\" "D:\newservershare" /MIR /NDL /NFL /MT:2
This will start a multi-threaded mirror of your old share to your new share, and it will be much more efficient than DFSR on the LAN. Run it once to get the majority of your data, and then whenever your scheduled migration is, run it again to get any changes (that's what the /MIR is for).
This way your second run of robocopy will be very fast, and you can cut over.
Also, if you're not already using a DFS namespace for these shares, I highly recommend you start now with this migration. It will make any future ones a breeze, as you can simply change the folder target with no change on your user's part.
In light of your first comment, I would still recommend pre-seeding for DFS with the following excellent blog post written by Ned Pyle with the DS Team at Microsoft:
Follow the Robocopy instructions, as this will make the initial replication MUCH faster.
I would still recommend using a single replication group for simplicity, unless your shares containing the 1TB of data are not grouped under a parent folder that can be replicated in whole.