IB will require that AA reissue the ticket for your flight. This takes time and effectively eliminates standby (AA cannot reissue unless the U class inventory is available). So you have to contact IB to get U released, and then AA to get the ticket reissued (presumably it is an e-ticket, which makes it a bit easier). IB seems rather chaotic in its ground handling and would probably prefer to load non-revs than release U inventory. At least that is my experience. IB can be incredibly bureaucratic, it once took me 2 hours to change a full-fare J ticket to an earlier flight on IB (same route and even the same date, just different time, and this was in MAD).