The correct procedure is:
1) drop the Y-pipe under the transmission pan,
2) unscrew the transmission pan bolts to let the fluid drain out,
3) pull the pan and gasket,
4) replace the filter,
5) clean the magnet,
6) reinstall the pan (the gasket is reusable assuming it is in good condition) and reattach the y-pipe,
7) slightly overfill the transmission through the fill port on the side,
8 ) get the transmission temp up to 167-176F by doing converter stalls (10 seconds running the engine up to 1500 rpms with the brake applied, followed by 10 seconds off idle - repeat until the temperature is in the range),
9) unscrew the fluid level check bolt with the transmission temp in this range and let the fluid drain out until there is only a steady drip coming out,
10) reinstall the fluid level check bolt.
If fluid does not come out at step 9, turn the engine off, let the area around the transmission cool down, add additional fluid through the fill port, and repeat steps 8-10.