I find it interesting that when you went to change your seat, it lost your original coach seat assignment online.

I had the same problem when I was traveling with a companion on a 9+ hour international flight.

Clicked on "change/view seat assignment" online, my seats showed up (day of the flight). I clicked on "return to itinerary", my seat assignment was GONE. (I had beautiful seats in the second row of coach)

Called CO right away. "I'm sorry those seats are held for airport checkin". This was 12 hours before the flight. Nobody could reassign the seats!

Got to the airport 3 hours in advance. My seats were gone. Flight was full, wound up (fortunately next to my companion) in a lovely NON RECLINE seat for 9 hours.

Their automated systems get better each day!

Maybe one day I'll get lucky and one of their E Check-in machines will print me out a boarding pass for EWR-HNL (Newark to Honolulu) instead of EWR-BOS (Newark to Boston)