It is difficult to say since the language in the owner manual seems to be ambiguous. ?Changed? is not very precise, but it is likely that it means a full replacement of the fluid. It is rare today, to find a torque converter that can be drained and if you can?t, you leave more than half the fluid in there. Even with a drain plug there is a lot left in there. Just dropping the trans pan and refilling is like paying off your credit card by making the minimum payment every month.
The bet thing to do is use a machine or just pull the cooler line(s) and pump the fluid out. Then drop the pan, inspect it, clean it and change the filter. Put a 2-4 (depending on capacity) quarts of fluid in the trans through the fill tube and pump half of that out. You just flushed the transmission. Button everything up and refill.