Transmission flush

You would be better off doing a "spill and fill". Basically removing the pan and draining it, then changing the filter. Then refill with new fluid. Reason being that unless you have a pump to push the fluid through the system you will need to keep dumping fluid into the transmission fast enough to keep the pan full so the pump can pull it through the filter.
Neither method exchanges all the fluid however, nor does a power flush normally.
There will always be "old" fluid mixed in with the new. With the pan method you get to change the fluid and the filter. You will not get the fluid in the converter but if that is a big worry you could install a pan with a drain and just change the fluid a few times after you change the filter.