robocopy source dest /E /ZB /COPYALL /R:1 /W:1 /MT:10
to get the first load over and then
robocopy source dest /E /ZB /COPYALL /R:1 /W:1 /MT:10 /MIR
until the cutover
This is a very simplified version, which you can extend to fit your specific needs. Be careful with /PURGE and /MIR options!
Hope this helps,
Peter