Dropping the pan and just replacing what fluid is lost after installing a new filter "usually" does no harm--it's when someone tries flushing the transmission with one of those vampire machines at quickie lube places that often loosens up crud and sends it throughout the entire transmission,where it can plug up the valve body or restrict fluid flow enough to make the clutches slip,then burn out..
Those places use those machines to save time and they do not bother to drop the pan or replace the filter (also many newer vehicles have no filter,just a screen, that isn't normally changed like the old days)...
Taking the pan down carefully and looking for any metal bits and clutch material is a good way to check the condition of the transmission,chances are if you find a lot of brass powder or steel glitter,or sludgy fluid--it'll most likely be giving major grief sooner than later..but I've had a few automatics that were full of junk in the pan,so much the pump was starved for fluid ,it clogged up the filter--and installing a new filter & fluid brought them back and they lasted as long as I owned the vehicle..others died almost immediately afterwards..
