The main reason people recommend not changing the fluid has to do with the clutch packs. The concern is that over time the friction material will wear away (this is also what becomes part of the black gunk in the pan). There is some truth to the fact that old fluid could varnish or gunk up the clutches enough to slow the transmissions decent into failure and slipping. The junk makes up for the lost friction material. I believe this a little bit, but if this is completely true it is only slowing the failure.
The fear is that new fluid, with its new clean oil, will erase any gunk that is helping the clutches. Basically you change the fluid, and it a short time the transmission starts slipping to death.
What is never mentioned is that the bad fluid will leave deposits and varnish everywhere and not lubricate as well... It can heat up more, causing faster breakdown of the clutches.
The way around this is a slow approach to changing the fluid. This is what I did to a friends Mazda and Chysler minivan (both over 100k with no changes). Get a oil vacuum/pump... Pull about 2 qts out of the dip stick/fill pipe. Use new spec fluid to fill it back up. Drive about 50 to 100 miles and feel for slippage.. If you don't feel any try it again. If you do this about 3 or 4 times and don't encounter slippage, you are ready to do a drain and fill.
Get a filter, fluid, and gasket and make the change. You will want to use solvent to clean the pan and pan magnet well.
If you are lucky you get to change about 60 to 80% of the fluid and clean the pan without killing the trans. If not, well you know where the transmission is health-wise, and at least you will know it without being stranded somewhere.
