List @@ sum
(* {a, -b, c, d} *)
{a, -b, c, d}
From the docs on Apply (@@):
f@@expr replaces the head of expr by f.
So List@@sum replaces Head[sum] (that is, Plus) with List.
You can also get the same result by changing 0th Part of sum (which is its Head) to List:
sum[[0]] = List; sum
{a, -b, c, d}