>>>>Under the earlier flight, though, it says "Seat Not Assigned".

Whenever I take a bump (so my booking and seat are given away),and then get confirmed with seat assignment on a later flight, my bump flight still is reflected in my itineraries (even 24 hours after the flight has departed) and is designated with the "seat not assigned" label. As the previous poster said, it is more a "marker" of having been involved in some transaction related to a flight than it is indication of confirmed space on the flight.