Lexus ES300 Trans Fluid

[Greenie]

My wife's '94 ES300 is due for a transmission fluid change, however the dealer and a reputable Lexus repair shop only recommend changing out a small portion of the fluid ie: what is in the pan (approx four quarts) at a cost of $120. They also do not recommend removing the pan or cleaning the screen. The transmission has been serviced as per the Lexus schedule.
The sevice writer says the transmission holds fourteen quarts so that would mean ten quarts would substantially dilute the clean fluid.
The car has 189K on the clock and they say any drastic change of all the fluid would be risky. I suspect their fear of liability is overiding practicality or is there a real risk with a vehicle this age?
I would like to change it myself using the same procedure I used on my Tundra (with Mobil 1 synthetic or even Dexron II) but I am hesitant to make a major transfusion now that these Lexus shops do not recommend it.
This car has been trouble free with the exception of normal maintainance items towit: tires, battery, tune ups, timing belts and so on. I know nothing lasts forever but this car does not use oil and has no fluid leaks and the transmission shifts the same as the day we bought it.
Any suggestions?