I'm assuming the American airlines vouchers were monetary vouchers and were how you paid. In which case it's as if you paid cash and it doesn't effect how you can or can't change the ticket.

If you bought a cheap return ticket then you're probably stuck. Only AA's more expensive tickets tend to be refundable or changeable. Your ticket conditions should have been provided. It can be a bit confusing but without them it's not possible to say what you can/can't do. If you can't understand them, call AA - or perhaps post the conditions quoted to the AA forum.