I haven't tried this recently, but it hasn't worked for the last couple of years. Maybe 6 years ago with a DONEx issued by AA I was able to simply change a LAN flight to the following day with a 2-minute call to LAN. A few years ago, however, LAN told me that now it had to be AA that made the change, and AA then said it had to "reissue" the ticket (all for a simple date change... not even a flight number change). This was especially annoying because the AA RTW desk kept US "banker's hours" and no other AA agent would touch the record. (AA has somewhat extended the hours of that desk, but still...)

Why simply rebooking a freely-changeable segment from a D class seat Wednesday to a D class seat Thursday on the same flight now requires a call to the originally ticketing carrier and a complete ticket reissue is not clear to me. It wasn't always that way.