You have to understand how an automatic transmission works, in order to understand whether whether the recommendation never to change the fluid makes sense. This recommendation is as old as automatic transmissions themselves. It's voodoo, and dangerously bad advice. More transmissions fail due to lack of fluid maintenance than any other cause. The fact that there are some (and I have one) with multiple thousands of miles on them is no reason to suspect that dirty fluid is the cause for their longevity. The more likely reason is that they were either built very tightly to begin with, treated very well during their service life, or both.
An automatic transmission works by using a stack of rotating clutch plates alternately sandwiched between a stack of non-rotating steel plates. The friction material on the clutch plates must contact as much surface area on the steels as available (that is, the full width of the plates) in order to provide the clamping force necessary to stop the clutch plates from spinning, and thus engage any particular gear. The less clutch friction material actually adhered to the plates, the less the clamping force. Period.
The purpose of ATF is primarily to enable the separate gear assemblies to function on call. Its secondary purpose is to provide cooling to those clutch plates and steels. ATF provides absolutely no friction benefit. As a matter of fact, loose friction material suspended in ATF actually prevents the clutch plate surfaces form contacting the steels. There simply isn't enough clamping force in the hydraulic system to overcome the mass of suspended particulate, so the clutches slip. Dry clutch systems work all over the world; they don't need ATF. They work because the friction material on the clutches has sufficient grip to grab onto the pressure plates. When they lose enough material, they slip. An automatic transmission works the same way, they substitute hydraulic fluid pressure for a clutch pedal.
There is absolutely no risk of damage to a modern automatic transmission from replacing fouled AT with clean fluid. Worn clutch plate slip, to one degree or other. They can't do anything else, and every clutch plate wears every time it's engaged. That's the way they work. They are supposed to be sacrificial components.
Worn valve body pistons, and their bores, will not be helped by suspended clutch plate friction material in there bores. If this could happen, there would be a huge market for ATF with friction material in it to "restore" failing transmissions. There isn't.
A failing torque converter cannot be hastened to its death by a fluid change. It's a pair of fans sandwiched between a couple of bearings. They fail because they cannot maintain internal pressure and the fans become unbalanced, or simply lose their efficiency.
