I have a 4R70w in my Ford Expedition, and it has about 170k miles on it. This is Ford's heavy duty overdrive trans, and I have always used a very large stacked plate external cooler. The first few times I had a dealer flush the transmission (which was recommended by Ford as the right service), and then at about 150k I dropped the pan and replaced the filter. The filter did not appear to be very dirty, despite the mileage.
Flush/fill/change transmission fluid doesn't seem to have any negative effect on my high mileage transmission. However, its been done about every 40k miles, which is different than trying the same thing on a transmission that has been neglected for years.