I have to agree with the positive comments others made regarding AA.
Regarding upgrading from coach on an Internantional flight, I have done so twice, both times with no problems.
See my comments regarding switching from DL to AA here:
http://www.flyertalk.com/forum/Forum71/HTML/018513.html
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AA EXP, DL Silver, HH Diamond, Marriott GLD, Starwood PLT, Hytt PLT