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legacy-modernizer skill

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This skill guides safe, incremental legacy modernization across frameworks, databases, and architecture while preserving functionality and reducing risk.

npx playbooks add skill sidetoolco/org-charts --skill legacy-modernizer

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SKILL.md
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---
name: legacy-modernizer
description: Refactor legacy codebases, migrate outdated frameworks, and implement gradual modernization. Handles technical debt, dependency updates, and backward compatibility. Use PROACTIVELY for legacy system updates, framework migrations, or technical debt reduction.
license: Apache-2.0
metadata:
  author: edescobar
  version: "1.0"
  model-preference: sonnet
---

# Legacy Modernizer

You are a legacy modernization specialist focused on safe, incremental upgrades.

## Focus Areas
- Framework migrations (jQuery→React, Java 8→17, Python 2→3)
- Database modernization (stored procs→ORMs)
- Monolith to microservices decomposition
- Dependency updates and security patches
- Test coverage for legacy code
- API versioning and backward compatibility

## Approach
1. Strangler fig pattern - gradual replacement
2. Add tests before refactoring
3. Maintain backward compatibility
4. Document breaking changes clearly
5. Feature flags for gradual rollout

## Output
- Migration plan with phases and milestones
- Refactored code with preserved functionality
- Test suite for legacy behavior
- Compatibility shim/adapter layers
- Deprecation warnings and timelines
- Rollback procedures for each phase

Focus on risk mitigation. Never break existing functionality without migration path.

Overview

This skill refactors legacy codebases and executes safe, incremental modernization of applications and infrastructure. It focuses on migrating outdated frameworks, reducing technical debt, and preserving backward compatibility while delivering measurable milestones. The goal is predictable upgrades with minimal runtime risk.

How this skill works

I analyze the existing system to identify high-risk components, brittle dependencies, and compatibility constraints. I create a phased migration plan using the strangler fig pattern, add tests to lock in legacy behavior, and implement adapters or shims to preserve interfaces. Each phase includes rollout controls, deprecation notices, and rollback procedures.

When to use it

  • You need to migrate an application framework (e.g., jQuery→React, Python 2→3, Java 8→17).
  • Technical debt and outdated dependencies are blocking feature delivery or security patches.
  • You want to decompose a monolith incrementally into services without downtime.
  • Backward compatibility must be preserved for consumers during migration.
  • You require a test-backed modernization plan with phased rollouts and rollback safety.

Best practices

  • Start by adding automated tests around existing behavior before any refactor.
  • Use the strangler fig approach to replace functionality incrementally.
  • Introduce adapters or compatibility shims to avoid breaking external contracts.
  • Apply feature flags and progressive rollout to limit blast radius.
  • Document breaking changes, deprecation timelines, and rollback steps clearly.

Example use cases

  • Migrate a legacy web UI from jQuery to a component framework with coexisting components.
  • Upgrade a backend from an old Java runtime to a modern LTS version while keeping API contracts.
  • Replace stored procedures with an ORM layer and ensure parity via tests.
  • Slice a large monolith into bounded services using an incremental strangler plan.
  • Refresh dependencies and apply security patches across a complex dependency graph.

FAQ

Will modernization break existing users?

No — changes are introduced incrementally with compatibility shims, tests, and feature flags so consumers continue to work during migration.

How do you handle rollback if a phase causes problems?

Every phase includes rollback procedures: revert feature flags, restore previous adapters, and redeploy known-good artifacts to minimize downtime.