normally changing the atf will not hurt anything if it's a pan drop and then idling with the cooler line disconnected to evacuate fluid from the torque converter and external cooler. you are not forcing fluid anywhere it has not been in the last 100k miles under the same pressure.
the misconception about high mileage fluid changes is when the transmission is already broken. people have slipping or weird shifting or some other abnormal operation and think that a fluid change will fix it, then when the tranny fails shortly after they falsely conclude the fluid change caused the problem.
the only truth to tranny fluid change causing a problem is when a shop uses a machine to backflush the tranny under pressure, often this was a money/time saving technique where the guy doesn't even get his hands dirty. the pan is never removed and they back flush the filter even and never change the filter. in that case problems did arise, and you can look up archived articles about it, at places like motor magazine i think. the practice was conjured up by folks selling the machines and advertising a faster/easy way to save a shop money along with 'good results'. naturally not every one in the automotive market is honest, and that practice has somewhat faded or was at least acknowledged when a pattern emerged.
but doing a normal transmission fluid change is not going to hurt anything.
using fresh atf that lubricates properly, rather than existing burnt worn out atf that isn't lubrication nor providing necessary friction for clutch operation, is not going to cause a problem. people simply draw the wrong conclusion on something they don't necessarily understand.
case in point, i was asked to help do a tranny fluid change on a ford aod trans in a grand marquis or some other full size car. relative drove it and my first thought is wow the car is still running for you knowing how it was cared for. point of the tranny fluid change was it had 150k or more miles on it, and the idea of having it changed was because 'shifting seemed a little off' they said. well the damage was already done being 150k miles. i dropped the pan and found bearing retaining ring or cage pieces of metal in the pan, along with brownish fluid but it didn't smell off. they swapped trannies from other car within that year, don't know if that one ever actually failed. did the tranny fluid change cause a problem? no. did it lead to tranny failure? no. did it reduce the life of tranny failure? no.
if your the original owner and know the history of operation on the vehicle and feel mostly certain no problem currently exists, then changing tranny fluid on a high mileage transmission in a normal manner will not cause a problem. it is not logical. continuing to run on existing 100,000+ mile atf that is worn out however will allow problems to eventually happen.
