Replacing or adding network protocols at runtime is problematic – it must involve synchronization of the protocol switch with ongoing local and network communication. We define a formal mathematical model of dynamic protocol update (DPU) and use it to define two DPU algorithms. The algorithms are based on fully-synchronized and lazy strategies. The two strategies implement updates with respectively, strong and weak safety properties. Our model allowed us to express the properties and the DPU algorithms clearly and abstractly, aiding algorithm design and correctness proofs.