Actually, ATF contains little, if any, detergents. It's usually the chemical flush that is used in some shops that cause the problems. I would just do drain and refills. Keeping that old, additive depleted fluid in there, is just going to kill you tranny quicker.
For the detergent in ATF:
http://theoildrop.server101.com/cgi/ultima...ic;f=5;t=002977