Service Oriented Architecture (SOA) is a recently emerged enterprise architecture promising to perform business processes efficiently and effectively. Service Providers and their customers negotiate utility based Service Level Agreements (SLA) to determine costs and penalties based on the achieved performance levels. In this work, we propose to enhance the flexibility of the WS-Agreement specification, a Web Service protocol to establish agreements on the QoS level to be guaranteed in the provision of a service. This protocol is generally aimed to be a "one-shot" interaction, i.e., the WS-Agreement is created based on a creation offer; the created agreement can not be modified and is effective until all activities pertaining to the agreement are finished or until one of the signing party decides to terminate it. Our main contribution provides for the integration of new functionality to the protocol that enable the parties of a WS-Agreement to re-negotiate and modify its terms during the service provision.