As you can see, there are a number of questions that need to be answered before any migration plan is setup and executed.
I personally planed and executed a migration from Windows 2003 SBS to Windows 2008 Standard edition and I can tell you that its a procedure that needs to be planned carefully because of the implications and problems that can arise if anything goes wrong.
Because I feel that you'll be performing quite a bit of a research before taking any action, what I can do at this point is to strongly advise you to take a backup of every server involved before performing anything.
It might also help you and other to explain how I went about my migration backup and testing method.
We used Symantec Backup Exec System Recovery to obtain a full image of the SBS2003 server. With this 45GB image, I used the VmWare Converter to convert the backup image to a VMware compatible file.
I then loaded the converted file into VMware workstation and worked the whole process using VMWare. Obviously I installed a Windows 2008 Standard server in VMware and followed the necessary steps for the migration.
I performed the migration 5 times to ensure I would capture all possible errors and everything went fine. When the day came to perform the migration on the actual machines, we took another backup of the SBS2003 just before we started and then went along with the procedure.
As expected, everything went fine, just as it did with the simulation.
If anything would go terribly wrong, we would be able to restore the server back to its original state before we started the migration.
If anyone else has followed similar or other techniques, it would be great to hear them.