Just tried changing my seats on my return segments tomorrow. Even when I clicked on the seat for the flight I wanted to change from my itinerary it took me directly to the outbound segment which was already completed and popped an error stating I couldn't select a seat for this flight (Duh!). Then I go to my actual return segments and it magically shows my seat as unassigned in the selection box instead of my actual seats from the itinerary details. I click back to avoid confusing the system and magically my seat assignments are still there. Ironically if I use the Delta iPhone app I can make selections just fine.