Legacy Modernization Pattern — Problem

Pattern: Legacy Modernization
Component: problem.md
Version: 1.2 | Updated: 2026-07-29

Statement

Teams announce a rewrite without characterization tests or a strangler boundary, then discover parity debt after the old system is already half-deleted.

Measurable symptoms

Symptom	How you detect it
Rewrite thrash	Behavior diffs found late by users
No seam owner	Traffic straddles old/new with no boundary map
Doc archaeology	Agents drown in unranked legacy manuals
Big-bang cut	Flag day replaces incremental slices

Root cause

Modernization is sold as greenfield speed. Without golden tests and a owned strangler seam, every slice invents behavior.

What breaks when this pattern is skipped

Parity incidents after each release
Rewrite estimates explode mid-program
Two systems diverge without an owner

Non-goals of this pattern

Not a pure data-center migration (use Migration)
Not drive-by cleanup unrelated to the seam
Not deleting legacy paths without characterization proof

Failures — recovery table
Workflow — ordered steps
Checklist — exit criteria

Changelog

1.2 (2026-07-29): Legacy Modernization problem page expanded with skip-impact and operator symptoms.