2002 Dodge Durango Changing Transmission Fluid

I recently got some Amsoil transmission fluid. The guy who sold it to me gave me an unorthodox way of changing the fluid that I have yet to see online and wanted to double check before I did it. My Durango holds about 15 quarts of transmission fluid. He said to remove the outlet to the transmission cooler and attach a rubber hose to it, leading to an empty bucket. Then turn the truck on in park, and let the fluid pump into the bucket (straight out of the cooler) until the fluid stops, then immediately turn the truck off. While the truck is off, replace with the same amount of new fluid that came out of the cooler. Then turn the truck back on, and let more fluid pump into the bucket while at the same time replacing with new fluid until I have reached the designated transmission capacity (15 quarts). Then turn the truck off, reattach the outlet cooler line to the cooler, and I'd be good to go. I just wanted to verify if this was safe before I did it. The only moving part in a transmission I am aware of (in park) is the transmission pump. Will doing this method hurt the pump or anything else? Would this method ensure I get mostly Amsoil fluid in the transmission or would it cause a considerable mix of old and new fluid? What would you recommend?