In a SOA environment services are implemented on top of resources across different administrative domains, in which IT infrastructure and internal resources are managed according to autonomous policies. Service Providers and their customers negotiate utility based Service Level Agreements (SLAs) to determine costs and penalties based on the achieved performance levels. In B2B scenarios where several service providers are involved in the composition of a service, and each of them plays both the parts of provider and customer, several one-to-one SLAs need to be signed. In such a rigid context the global QoS of the final service can be strongly affected by any violation on each single SLA. In order to prevent such violations, SLAs need to flexible during service provision. The WS-Agreement specification, developed by the Global Grid Forum, is a Web Service protocol to establish agreements on the QoS level to be guaranteed in the provision of a service. The committed WS-Agreement can not be modified during service provision and is effective until all activities pertaining to it are finished or until one of the signing parties decides to terminate it. In this work we focus on this specification and propose to enhance the flexibility of its approach. We integrate new functionality to the protocol that enable the parties of a WS-Agreement to re-negotiate and modify its terms during the service provision, and show how a typical scenario of service composition can benefit from our proposal.