Full drain and flush. The body itself of the trans holds only about 3-5 quarts of fluid, while the torque converter holds more. Replacing only half the fluid is a waste of time and money, because as soon as you start the vehicle, you have nothing but dirty fluid, even with a new filter.
A fluids (trans, differentials, power steering, brake, t-case, etc.) change is a very good idea every 5 years or so, as vehicles last for many more miles now than they did in the old days, and while oil doesn’t wear out, it does get contaminated and the detergents and additives do end up being used up.
JMHO, YMMV.
Me, I wouldn’t flush. Tried that once and lost the Tranny 20k miles later. Drop the bottom and let the crap out no problem. A flush pushes all that crap around even more.
Some Fords had drain plugs in the torque converter. At the Ford garage we had a torque converter cleaner, you’d pull the plugs, lock it down, put a hose in the snout and hit the switch. It would pump solvent in and rock it like a paint shaker. Rotunda!
This^.
I always do a trans full flush and filter replacement on all my vehicles between 75,000 and 100,000 miles, even on my vehicle where the manufacturer declares the fluid is "good for the life of the transmission." Yeah, when the trans blows up at 175,000 miles then they say, "See, that was the life of your transmission!"
The tip here is not to wait too long to do a flush and fill. If you wait too long, your transmission could start slipping after servicing.