The company plans to make the changes next month, so that drivers will be responsible for closing doors.
Southern says there will be no job losses, and no-one will take a cut in salary, while the union believes conductors should retain a safety critical role.
A Southern spokesman said: "We welcome the suggested suspension of industrial action, but we don't need three months to resolve this.
"We are ready to sit down with the RMT and discuss a way forward that we believe that they, our employees and customers will welcome, and can bring an end to this dispute.
A map of disruption to services supplied by Southern Rail
Southern
"In the meantime we would ask the RMT to work with us, as previously requested, to address the main cause of the current service problems, which is the remarkably high levels of sickness amongst some RMT members."
