you can't totally drain an automatic (torque converter too). well most modern sealed ones. so you drain out as much as you can, most mechanics have a device to suck out some of the old stuff, then you can fill it back up with the similar amount that you took out, replace filter yada, you use the vag-com to measure the temp, at a specific temp the fill-hole (sealed trannies) will be correct, only at that temp which is not cold and not fully warm. (hence vag-com to read trans fluid temp).
to do a really good flush would probably require you to repeat this several times with some driving in between. I just stick to 2y/24K intervals and have a link to a place that does reman'd tiptronics for $2695+core which really is about what any tranny would cost remanufactured with 2/24K warranty. labor is the bitch part.