You should not have any trouble as long as you keep the voltage
constant, i.e., replace the six 1.2V NiCd cells with six 1.2V NiMH
cells. There should also not be a problem using the same charger to
recharge the NiMH cells. Of course, the benefit to you is a longer time
between charges because 3300 mAh will last 2.5 times longer.