Real-time networks based on Ethernet require robust quality-of-service for time-critical traffic. The Time-Sensitive Networking (TSN) collection of standards enables this in real-time environments like vehicle on-board networks. Runtime reconfigurations in TSN must respect the deadlines of real-time traffic. Software-Defined Networking (SDN) moves the control plane of network devices to the SDN controller, making these networks programmable. This allows reconfigurations from a central point in the network. In this work, we present a transactional model for network reconfigurations that are synchronously executed in all network devices. We evaluate its performance in a case study against non-transactional reconfigurations and show that synchronous transactions enable consistency for reconfigurations in TSN without increased latencies for real-time frames.