Tonight I changed an existing reservation. I moved the return segment of a round trip out one day. When I went to select my seats for the new flight, I was first asked to select a seat for the outbound flight, even though I had already done that when the original reservation was made. It showed no seat selected for the outbound flight. The seat I had previously selected was not selectable. I selected seats for both flights. I then was able to go back in and get back my original seat on the outbound flight through the normal View/Change Seats option.