I didn't care for the Dorman pan. I looked at it at Advance, and walked out without it. The sealing lip didn't give me a warm, fuzzy feeling. Might be fine, but I didn't want to chance it.
I would try to change all the fluid at once. Doing a little at a time, you will end up throwing away a lot of your new fluid, would take 20+ quarts. If you can get under the car and drop the pan, you can also drain the torque converter. Job done.
Don't forget the filter, and either re-use the stock gasket or buy another Ford gasket. Those thin rubber ones that come with the filter aren't any good.
Al