cannibal... i think that even though you want your system to be flexible as possible, most people will have at least one switch planned during a thru hike of the at.
sgt rock was talking about that this week end at hot springs. you may want to go to his site (hiking headquarters) & ask the same question of him.
but keep in mind that he's out doors a lot & seems to be a little more warm blooded than most. this past week end as the temps dropped, he was one of the few that stayed w/ shorts & a tee shirt most of the time<g>.
i would personally lean toward an under quilt & a 3/4 length ccf pad combo.
this same combo would probably be lighter for the warmer stretches & thicker for the colder times. ...tim