A configuration controller (60) is provided to make changes to configuration of a communications network (10) having VNFs used in providing services to UEs. If the change could affect a service using a first VNF (30), if made while that VNF is in use, a substitute VNF (32) is instantiated corresponding to that first VNF. The first UE is transferred to use the substitute VNF instead of the first VNF. The proposed change to the configuration is made with respect to either one of these VNFs, while the service to the first UE is maintained using the other one of these VNFs. If needed all VNFs and a service area can be substituted. Thus, such configuration changes no longer need to await a quiet period when few or no UEs are attached, and enable the configuration changes to be made more rapidly, responsive to changing circumstances.