I have a similar question. I'm booked on a LAN ticket in business class (I) from EZE with a stop in MIA and then transferring to AA metal but on an LA flight number to LAX. My flight lands MIA 5 hours before the scheduled AA (LA codeshare) to LAX and there is a flight 3 hours later, also on the 777 that i'd like to take instead but it doesn't have an LA flight number so I'm not able to book it now (fare goes crazy). Once I start travel or even check in at EZE, can I change this (as opposed to standby) to the earlier 08:25 AM flight instead of the 10:45 AM flight I'm booked on? This would also be key because of the (unlimited free) upgrade possibility to First on that flight segment.

I'm new to AA, so any advice is appreciated.

Thanks!