See the migration path before you write the cutover runbook
A safe migration moves through mapping, shadow reconciliation, and then a controlled boundary where rollback is still possible.
A migration path customers and finance can live with
Safe migrations run the new engine in shadow mode first, compare outcomes, and cut over only after the team can explain the differences it sees.
Document plan logic, proration, discounting, and webhook-driven state before writing cutover code.
Send the same lifecycle and usage inputs into the new engine, then compare outputs by customer and billing period.
Move cohorts on a clean boundary with rollback preserved and historical invoices left intact.
Step 1: map the billing behaviour you already have
Before you write migration code, define how plans, prices, discounts, entitlements, and webhook-driven transitions behave today. Hidden incumbent behaviour is usually where revenue surprises begin.
- Document proration, rounding, and invoice finalisation rules.
- List which webhooks or operator actions move subscription state today.
- Identify the source of truth for customers, subscriptions, and usage.
Step 2: mirror inputs and reconcile outcomes
Mirror lifecycle events and usage into the new engine. Produce invoice previews or shadow invoices, then compare them against the incumbent by customer, billing period, and line-item category.
- Use idempotency keys so mirrored events can be replayed safely.
- Generate evidence, not charges, while the new engine is still in shadow mode.
- Track differences in a way finance and support can review quickly.
Step 3: cut over on a boundary you can explain
- Move new customers first, or choose a cohort by renewal date.
- Keep a rollback plan that re-enables the incumbent path if anomalies appear.
- Preserve historical invoices and exports instead of deleting evidence after launch.
The strongest teams treat cutover as an operational event with owners, runbooks, and success criteria, not just a deployment.
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