Long migration planner

What It Does

Delegates the exploration phase of a big migration (framework upgrade, language switch, DB swap) to the Plan subagent. Returns three strategies — big bang, strangler fig, parallel implementations — each with risk, timeline, and rollback notes.

When It Triggers

• ▸"Plan the migration from Flask to FastAPI"
• ▸"How do we upgrade React 17 → 19?"
• ▸"We need to move off MongoDB — plan it"

SKILL.md

---
name: long-migration-planner
description: Plans multi-sprint migrations (framework upgrades, DB migrations, language rewrites) using the Plan subagent to explore options without polluting the main context. Use when user says "plan this migration", "upgrade from X to Y", or "how do we migrate".
---

# Long Migration Planner

## Steps
1. Delegate to Plan subagent: "Survey current usage of [library/pattern]"
2. Ask Plan subagent to produce 3 migration strategies:
   - Big bang · Strangler fig · Parallel implementations
3. For each strategy: risk level, estimated weeks, rollback path
4. Main agent synthesizes: recommended path + phase-by-phase checklist
5. Save the plan to `docs/migrations/<target>-migration.md`

Why Plan Subagent

Surveying current usage can touch hundreds of files. Keeping that exploration inside a Plan subagent means the main context stays focused on the strategic decision, not the raw survey data.

Gotchas

• ▸The Plan subagent cannot execute destructive actions — it only surveys.
• ▸Save the final plan to disk so it survives across sessions.
• ▸Revisit the plan after phase 1 — real data from phase 1 usually invalidates assumptions in phase 3+.