Try calling LAN directly. Just yesterday, I wanted to change the date of a LAN GIG-SCL segment I have for this June. Although I could see A availability on AOW, AA RTW could not get the seat. This was in the afternoon and my AAngel works mornings, but I was impatient and did not want to wait. So, I called LAN directly and they found the reservation, made the change and assigned me a seat with no problem. The change is even being reflected on AA.com. Just make sure that you call AA RTW afterward to cancel any reservation you may have for another LAN flight using the same segment.