Modifying quality of service treatment for data flows A method of transmitting a data flow via a network is disclosed where the network supports transmission of data in accordance with a plurality of Quality of Service, QoS, models. Prior to transmission of the data flow, a client system configures a first class of service for the data flow based on a first QoS model, and a first portion of the data flow is transmitted through the network in accordance with the first class of service. In response to detecting a renegotiation condition, the network communicates with the client system to configure a second class of service for the data flow based on a second QoS model, and a subsequent portion of the data flow is transmitted through the network using the second class of service.