Ah yes, this would increase the voltage from 7.2 V to 8.4V which is close to 9V. I'm not sure if the reqired voltage is 3x1.5 or 6x1.5 since it has 3 batteries in each leg.
Could this damage the electronics due to lower resistance of the batteries?
I wonder if there exists rechargeable batteries with 1.5 voltage, li-ion perhaps...