Direct answer
What this means
Safe legacy modernization begins by recovering and testing the system's actual behavioral contract. Teams should isolate stable boundaries, introduce observability, migrate in reversible increments, validate data explicitly, and avoid a full rewrite unless the existing system cannot support a controlled transition.
Start with behavioral discovery
Document users, interfaces, data flows, operational procedures, failure modes, and undocumented dependencies. Add characterization tests around behavior that must survive the transition.
Choose the smallest safe boundary
A route, workflow, interface, or service boundary is often a better first migration unit than an entire application. Smaller boundaries make comparison, rollback, and stakeholder validation practical.
Treat data migration as a product
Define reconciliation, repeatability, rollback, ownership, audit evidence, and the treatment of malformed or incomplete historical data before cutover.