It looks like I'll be replacing my evaporator so I figured I may as well replace the dryer at the same time. It looks like it needs to be done from the underside of the vehicle. Is it as simple as removing the protective cowling under the engine compartment and loosening the pipe fittings and pulling the contents out?

I'm not familar with the 03 forester layout, but you should be able to just remove the two lines going to the dryer, remove the pressure sensor, and loosen the clamp that holds it. You'll want to drain the oil out of the old dryer into a measuring cup, and put the same amount of oil back into the new dryer. It should use PAG 100.