Just recently I replaced all 10 qts in my 99 Forester by removing supply line to trans cooler(pax side connection to lower radiator). I filled trans pan w/ 3 qts, started engine, and pumped out 1-1.5 qts (only 10-15 secs), stopped, refilled trans pan, and dumped caught fluid. I repeated this until clear new fluid emerged from supply line at about 10 qts. Adjust fluid level by removing or adding to get to full cold position.
Above procedure leaves a small amount of old fluid from trans cooler and trans return line in the system. I thought of this after completing my work. Not sure how to recover this last bit of fluid, but it might be possible to suction out w/ vacuum hand pump or air gun vaccum.
If fluid is in good shape, say changed w/i last 30-50k miles and does not smell burnt or foul, drain and refill pan should be fine IMO.
See you down the ROW!
1999 Subaru Forester