[JimD]

Conventional automatics have a torque converter and torque converters do not, to my knowledge, have drain plugs. So if you pull the pan or pull the plug and change the fluid and hopefully the filter on a conventional automatic you are removing well less than all the fluid. The logic for gear changes in conventional automatics also used to be hydraulic in a valve body. I do not believe that is the way it is done any more. Horror stories about fluid changes in conventional automatics at high mileage makes many leery. The "flush" normally just involves circulating some fluid so the torque converter is changed too.

DCTs are very different and it seems to me more like manuals than automatics mechanically. They are automated manual transmissions. The gear side of them would need a fluid change like a manual transmission needs the fluid change. They have clutches, however, which conventional automatics also have, but I don't see why changing the fluid would harm a clutch. It is a key source of heat so the clutch would logically benefit from new fluid with the additives back at new levels. If I ever own a DCT, I would treat it like a manual. Might do an early change then changes about every 50K miles.