When I wanted to change my EZE-GRU-EWR ticket last year on my first 2 calls I managed to get a quite decent list of dates available. I wasn't sure when I wanted to return so I waited about a week for me to think, then when I called, the ticket went from 90 day validity to 30 days...
Then they suggested that I spend the time (about 3 weeks) enjoying myself in Buenos Aires.