Hexagonal architecture migration transforms a tightly-coupled Express monolith into a layered, testable system using ports and adapters. The BEFORE state shows direct HTTP routes and WebSocket handlers calling Prisma ORM; the AFTER state isolates use cases (SendMessage, CreateConversation, ListMessages, TransferConversation) behind port interfaces (MessageRepository, ConversationRepository, RealTimePublisher) with pluggable adapters for HTTP, WebSocket, Prisma, and Socket.io. This pattern decouples business logic from infrastructure, enabling independent testing via mocks and fakes, and simplifying expansion to new domains like Agents, Contacts, Teams, and Labels. Fork this diagram to customize the migration roadmap for your codebase, or download as .drawio to embed in team documentation. The six-step incremental plan—from use case extraction through adapter implementation to unit tests—provides a practical blueprint for teams migrating legacy Node.js services without downtime.