Getting from AA's part of B to A isn't too long of a walk (not much longer than the distance between some of the gates in A), just head for central parking but turn left when another causeway connects with yours just outside the garage and bear right to A when that causeway splits.

I ran from check in counter to check in counter in under 7 minutes once when my AA flight cancelled and they rerouted me onto a DL flight scheduled to leave before my original flight. The DL counter agent took over the ticket ~32 minutes before departure and my upgrade cleared at the gate.