I would just pay to upfare the outbound and the $50 fee for AA to take over the ticket to do so. Keep in mind I don't think you can just upfare one leg on a connection. And, you would be subject to married segment availability, so the price you're seeing for leg X-Y could be very different than the price you'd be quoted for X-Z. I have to ask, though, is X-Y in First worth all of this?