For those who don't know, most ATs hold about 7-10L depending on the car. However the AT oil pan only holds approximately 1/3-1/2 of the total oil capacity and the remaining is still in the AT galleries & torque convertor and 99% of the time you cannot get it out. So you need to do successive oil changes to purge the lot.
Best thing to do for the average DIYer is to drain the pan and fill every weekend for 1 month. During the week you drive the car so the fresh fluid mixes with the old. Then you have 4 successive changes to dilute the remaining oil.
You can do this every 40,000km or you can do 1 oil pan 'drain' every 10,000km.