Szymon Fiedler's wroclove.rb 2025 talk. Describes a three-month project at Lemonade (with Arkency) to replace the legacy HO4 renters-insurance quoting flow without breaking parity. Constraints: no UX regression, tight timeline, and business rules embedded in a years-old Rails monolith with sparse documentation and original authors gone. Methodology: treat existing system behavior as a specification; instead of writing full test coverage for dozens of microservices, sample 1–2% of production quotes, snapshot the quote model before/after underwriting, and record all HTTP interactions via Typhoeus callbacks (and a Net::HTTP override for AWS/S3). A verifier runs the new flow inside an ActiveRecord transaction that is always rolled back, stubs the recorded HTTP calls, blocks all other outgoing HTTP requests, and diffs the resulting quote against the old one (excluding ids/timestamps via an 'invariable_attributes' list). Background jobs are guarded with an after-commit wrapper so no side effects leak; thread-safety ensured under Unicorn with concurrent-ruby tasks forced inline. Sample collection and verification are decoupled to allow iterative polishing. The project shipped a gradual feature-flag rollout with no rollbacks; project leader 'Gaya' praised it as one of the best releases he had experienced.