A rewrite is a business-risk decision
Old code is not automatically disposable. It contains undocumented behavior, integration assumptions and recovery habits. Before proposing replacement, map runtime entry points, databases, jobs, authentication, external contracts and the flows whose interruption would affect customers or operations.
Create a safety perimeter
Characterization and integration tests capture critical observable behavior without pretending the design is already clean. Add logs and metrics where failures are currently invisible. Reproduce the production-shaped path locally or in a controlled environment before changing its dependencies.
Modernize by boundary
Upgrade a package, isolate a domain rule, replace an adapter or move one endpoint at a time. Compatibility layers are acceptable when their purpose and retirement condition are explicit. Small changes preserve rollback and make review meaningful.
Keep delivery states separate
Compiling, passing local tests, merging, completing CI, deploying and homologating are different events. A modern architecture on paper does not prove that traffic reaches it. Operational evidence and client decisions remain gates, especially in regulated or multi-tenant systems.