When is refactoring appropriate in a legacy system, and what precautions should you take?

• A. When code is hard to modify
• B. Implement broad rewrites to reduce risk
• C. Wait until a complete rewrite
• D. When code is hard to modify or test; ensure tests exist, make small incremental changes, and validate with regression tests

Answer: (D)

Refactoring a legacy system makes sense when the code is hard to modify or hard to test, because the goal is to improve structure and maintainability without changing how the system behaves. The best practice is to ensure there are tests before making changes, then apply small, incremental edits rather than broad, sweeping rewrites. After each change, run regression tests to verify that existing functionality still works. Broad rewrites increase risk in fragile codebases, and waiting for a complete rewrite is often impractical; you reduce risk by building test coverage and validating changes step by step. If testing isn’t in place yet, start by adding tests for critical paths, then proceed with careful, small refactors, continually verifying with regression tests.