Just a quick data point, I was just able to moved up my return dates a few days with no different in fares or taxes. I did checked on AA to make sure AA38 (MIA-LHR) has A class available though. There are some days that I wanted to travel that only has "First-Flexible" aka F fare. So guess I'll just spend an extra day once I get to CMN and then a one way trip from there to CGK to start on another deal that was posted on here, CGK-SFO for 950 euros
Oh, I used the AA's chat function on the website and this time, it took only 1 try... versus 4 the first time lol