I'd take a look at your fare rules, as most trans-con 2-cabin F fares explicitly forbid travel on three cabin planes. See for example KUPP7MZ (SAN-MIA). Perhaps you'd get lucky in that the agent wont look at the rules, but they could.
THE FARE COMPONENT MUST NOT BE ON
ONE OR MORE OF THE FOLLOWING
AA FLIGHTS 1 THROUGH 4
AA FLIGHT 10
AA FLIGHT 15
AA FLIGHT 16
AA FLIGHT 18
AA FLIGHT 19
AA FLIGHT 20
AA FLIGHT 21
AA FLIGHT 22
AA FLIGHT 30
AA FLIGHT 32
AA FLIGHTS 33 THROUGH 34
AA FLIGHT 40
AA FLIGHT 59
AA FLIGHTS 117 THROUGH 118
AA FLIGHT 133
AA FLIGHT 177
AA FLIGHT 178
AA FLIGHT 179
AA FLIGHT 180
AA FLIGHT 181
AA FLIGHT 185
AA FLIGHT 201
AA FLIGHT 299
AA FLIGHT 435
AA FLIGHT 1520
AA FLIGHTS 2800 THROUGH 2899.