If the transmission has been serviced before and/or has relatively low miles (under 100k), I would drop the pan, clean it out and replace the filter & gasket. Top off with with fresh fluid and then just do drain & fills every 30k going forward. IMO flushing the converter is unnecessary in most cases, as is replacing 100% of the fluid. A good freshen-up every so often will keep it going a long, long time. But if the transmission has never been serviced and it's a high mileage unit, I would just maintain the fluid level until it starts to give warning signs.