Hi James - I'm not sure thats what he wants to do. I think he wants to leave the mount path in place and just to move the data off it to another mount path (I'm guessing so he can remove\retire that mount path)
I have the same issue - although I want to move data off the mount paths of one MagLib to another MagLib. The only way I can see of doing it (from options in CommVault and documentation) is to associate the Storage Policy Copy writing to the MagLib to another and then disable all the mount paths for writing and then turn off 'Managed Disk Space' for the copy (or leave on and check the contents of the mount path are out of retention and then delete the contents)
Unfortunately the issue I have is that some of the data on the mount path has infinate retention and won't age off.