Sory to contradict you but it IS possible to drain and completly change the oil in the auto trany.
http://www.brickboard.com/FAQ/700-90...onAutoAW30.htm
This applies to other auto trans also. I've done it myself on a 940 & 960. Well worth the effort. I was told my converter was shot at 113,000 miles. Changed the oil and it's still going strong at 150,000.