OK... You guys are really into your myths! There is nothing wrong with flushing a transmission. If you flush a dieing transmission, it will die. If you don't flush a dieing transmission it will still die. The point is that you've waited too late and the transmissino is about to fail either way. Also, niether means of flushing a transmission "power flushing" forces fluid through your transmission at some super rate to clean it. Pump inlet flushing machines use the transmissions own pump to flush the fluid while cooler line fleshing takes the fluid comming from the torque converter.
Pump inlet flushing is better.
You can only change about 1/4-1/3 of the transmission fluid in toyota AT's by draining the pan. It normally takes 12-15 quarts to fill a Toyota AT.
trdcamry2003 is right on. If the fluid has metal flakes in it, the transmission is dead, it's only a matter of time b/c you've already ground it up. Otherwise, change it every 30,000m.