How many miles are on this fluid? Are you feeling any drivability issues? The odor is usually caused by the fluid getting overheated (or just old and needing replaced), the dark color is usually caused by friction material from the clutch packs (can also be normal on old fluid-the disks wear down over time so some fluid darkening is expected in old fluid). Depending on what side of the fence you are on, I'd go with either a flush to get all of the old fluid out of the converter, or some people say to only do a filter change and refill whatever comes out of the pan. You could always do both too.
