Just turn up for check-in with a little extra time. AA will take your one paper coupon and exchange it for a two segment eTicket. If needed just mention the magic words equipment change and involuntary reroute. By the way, you do not have to take the flight connection AA has given you, you can request a different one if you like; they will agree to any reasonable request, but if you do decide you want to change the connection it's better you get the ATW desk to change it before you get to the airport.