The trans is designed with lifetime fluid (like the coolant). That is why there is no TRANSMISSION FLUID LEVEL dipstick! That said, if you change the torque converter (or transmission) you will need to replace the fluid.
Pan does not have to come off to change ATF oil. The larger plug is for refilling and the small one for draining. Make sure to see if your refill(large) plug breaks loose first or your left with an empty tranny pan with no way to fill it. Only reason you need to take the pan off is if your replacing the filter otherwise just changing or flushing the system just requires the two plugs removed.