Have also dropped the trans pan, wash it out, put on a new filter, put it back on again, put the same amout of fluid back in that came out. Start it up, let the new fluid run through the converter again. Repeat that one more time to help replace the fluid in the converter with new.
The transmission shops also have the transfusion machines that tap into the trans cooling lines and pumps new fluid into the trans while letting the old fluid out. Trans guy says it takes a good amount of fluid till it runs clear. The pan does not come off and filter does not get changed, but at least it gives you a quick full system fluid change without dropping the pan.
Have had them do this to a couple of vehicles for me to get fresh fluid in them. When the fluid gets old and breaks down it starts to loose it lubricating qualities, then the parts start going away.
Ounce of prevention worth a pound of cure.