Nothing.  Your existing wiring is still good for this battery changeover.  Simply hook up your 12V batteries the same as your 6V batteries were hooked up.  However, you need to make sure you have heavy load batteries otherwise you may have to parallel connect two more with the series-connected batteries to carry the ampere load of  your motor.  If I was doing this I would add the parallel-connected batteries, one @ the plus of the motor and one @ the end battery to the motor negative.