Even if there wasn't a drain plug in the pan, you can still change the fluid without pulling the pan. Just remove the soft line from the fluid cooler that leads back to the transmission and stick it in a big bucket, then run the engine and add new fluid through the dipstick tube until you've pumped through ten-ish quarts. Hopefully you can pour in at about the same rate it comes out the tube, and the better way might be to have an assistant change the gearshifter (chock the wheels/set the brake first) while it's happening. You are able to almost completely change all the fluid that way - better than just draining it.

Bonus points for pulling the pan, cleaning the screen then replacing the pan first, though.

John B.