E450 trans oil change?

The best way to change fluid is from the output line from the radiator transmission cooler. Put a hose on it going into your pan and have your helper start the engine and when you see bubbles coming out of the hose, have them turn off the engine. At that point drop the pan and change the filter, there will be minimum fluid left in the pan at that point, minimizing the bath you would otherwise suffer. Clean the pan and put it back together, refill the transmission, and have the helper start the engine again. Repeat the "run until bubbles and refill" until the fluid starts coming through with the new, fresh red fluid color. During one of those cycles, have your helper hold down on the brake and move the shifter through it's different positions to make sure the valve body gets flushed with new fluid. Then, reconnect the cooler line and fill to the dipstick mark. This is the only way you will get a total fluid change including the torque converter. You will also avoid the mess of dropping a pan full of fluid if there is no plug to drain it. For many years Ford trucks had a short piece of hose to remove on that cooler output line for this exact reason.