On most transmissions (e.g. those without torque converter drain plugs) dropping the pan and changing the filter only changes a portion (variable) of the fluid. If this is done every 30K or so this isn’t a problem.
If one does want ALL of the old fluid out and ALL new fluid in then a fluid exchange can be done via machine - but should only be done AFTER a new filter (as noted).
If you are servicing it regularly, just service the filter and forget about it.