This was a good thread on this topic
http://www.flyertalk.com/forum/Forum71/HTML/009493.html
I always use ITN.net (expert mode) to get an ideal on the flight's capacity status. Or you can check with AA just a few hours before departure. No guarantees because as final numbers depend upon the number of no-shows.
Using ITN.net, I actually called the EXP desk, asked about the status of a Flt, which was confirmed as terribly oversold.
Offered to take an earlier flt, (leaving an hour and a half earlier)
EXP said that they could not guarantee the voucher but would put a notation in the record.
When I got to the MEM airport and inquired about whether they wanted me to fly earlier. The answer was yes and based upon the notation in my file, they gave me a $200 bump voucher.
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AA member since 1981