I figured that changing the transmission fluid was just normal maintenance once a vehicle got some miles on it. We changed the fluid on the wife's GMC Acadia and a couple days later I was listening to one of those car service gurus on the radio about changing the atf and he said that if it isn't broken, it doesn't need fixing and to leave it alone. Then a guy or two phoned in and said that their transmission started to slip shortly after they had changed out the fluid and cleaned the screen. It has been 20k more miles on the GMC since the change and no issues so far, but we are pretty easy on our cars. I was kind of surprised when a car fix it guy talked about NOT doing a $ procedure.
From another thread:
Old, high mileage fluid becomes somewhat abrasive with all the fine clutch material particles, and will actually help worn clutches to engage with out slipping. Sooo, what happens to a lot of high mileage transmissions that are serviced for the first time? New fluid, which doesn't have the abrasive particles, does have a fresh additive pack which is more "slippery", results in slippage because the clutches are likely worn and glazed. Not the mechanic's fault. I will not service a transmission in my shop with more than 90k miles on it, if I can't verify previous service at 40-50k miles. I'm not gonna be "that guy".
