Yes, you can call and change routing and date without a fee as long as origin and destination remain the same (there are other caveats but they won't affect your situation). If the itinerary shows on BA.com it's bookable at the saver rate, though sometimes ba.com doesn't show AA availability when aa.com does. If aa.com has saver availability (the light blue color for business vs the dark blue color), you can combine it with LAN saver seats to book your itinerary.