In ORD and as an EXP on an International flight .. with any guff from the Gate .. I would simply have walked into the Flagship and asked them to sort out the problem.

Granted.. other cities.. or others not being EXP would not have this option.

I think the fact the flight was oversold was the reason they were getting picky.

The computer flags customers who may not make the connection .. so the Gate probably already said 'they will not make the connection' and gave away your seats.

If you have names and want to complain to AA in writing. I would also write to the ORD Station Manager (I dont know the name) at:

American Airlines
Attn: Station Manager
O'Hare International Airport
Box 66065
Chicago, IL 60666

The original EXP agent may have made the mistake by protecting you on the later flight .. and the fact your LGA-ORD plane got clearance and arrived sooner than expected ..

You got on the later flight in the cabin you purchased .. right? Or was it an upgrade and the upgrade failed?

I'd hope by now you have cooled off enough. I seriously doubt the EXP would ever hang up on a customer .. have you tried calling back (could have been a technical problem rather than bad service).