I have my itinerary pulled up at delta.com (the "trip details" page) and there's no option to change my seat. The seat numbers aren't hyper-linked.

This is an award ticket booked well in advance, with some schedule changes since initial booking, possibly an equipment change.

Do I need to call Delta to get the seats changed? I do not want to sit in 20B if I can possibly help it. Is there a charge to do this over the phone?