Tranny flush question

Transmission return line on rad broke on lil bro's GC. He had an uncle help him with it, and they replaced the lost fluid they put the wrong fluid in. Now the trans won't shift at all. What should I do in order to get the wrong fluid out so that I can replace it? I know about pulling the pan and changing the filter, but don't I need to get the fluid out of the torque converter too? Am I going to have to pull the tranny off to do it myself? Thanks.

On some vehicles you can detach the cooler lines and allow the fluid to pump out of the line to the cooler (catch it in a pan so you don't make a mess) and on the other line, attach a length of flexable hose and allow it to suck up new transmission fluid directly out of a container . You continue this process until you see that the exiting fluid is now the same color as the entering fluid. Some transmissions do not pump fluid through the cooler while in park, you may have to put it into neutral for the procedure to work.