Some places just drop the pan (which you need to do in most cases to replace the filter), then top it off after installing a new filter/re-installing the pan. But this only removes about 4 quarts or so (average) from the system. There's still a lot of fluid in the torque converter, lines/cooler and the unit itself. To replace all the fluid, the system has to be flushed - which usually means hooking it up to a machine that sucks out the old fluid and replaces it with fresh fluid.
I've heard pros and cons for both approaches.
In my old cars, I just do the drop the pan/replace the filter thing every other year.
So far, no problems...