What would happen to the rod if the current were reversed?
If you reverse the direction of the current in the rod, you can still use Fleming's left hand rule. Your first finger is placed in the same direction as before, in line with the direction of the magnetic field (i.e. north to south).
The second finger is placed in line with the direction of current in the wire, the opposite direction to before.
Your thumb will now show you the direction of the motion on the carbon rod.
In this case it will be in the opposite direction to the first experiment.