A migration in eight steps
- Inventory call sites. Search every repository for the SDK, client and raw HTTP calls to the API. Include background jobs, webhooks and scripts, which are easy to miss.
- Map the changes. For each item in the changelog, list the call sites it touches and the expected behavioral difference.
- Pin your current version so nothing moves under you mid-migration.
- Isolate the client. Route calls through one adapter or wrapper so the change lands in as few places as possible.
- Update types and SDKs, then fix what the compiler finds.
- Verify behavior, not just compilation. Compare responses from the old and new versions for real requests.
- Roll out gradually behind a flag or per tenant, watching error rates and business metrics.
- Remove the old path and the flag once traffic has fully moved.
Verification is the step teams skip
Most migrations stop at step five: the code compiles and the tests pass. But tests usually run against mocks that encode the old behavior, so they cannot tell you whether totals are now in cents or a list now sorts differently. Replaying recorded production requests against both versions and diffing the full response envelope (status, headers, body, timing) is the most direct evidence that behavior is unchanged. See traffic replay testing.
Estimating the effort
Effort scales with call sites, not with the length of the changelog. One renamed field used in 200 places across 30 repositories is a bigger migration than ten changes to an endpoint you call once. Count call sites and owners before committing to a date.
If you are the API provider
Every hour your customers spend migrating is an hour they are not building on your platform, and a risk that they delay the upgrade indefinitely. The most effective providers reduce that cost directly: precise changelogs, Deprecation and Sunset headers, codemods, and increasingly, automated pull requests against customer code with evidence that behavior is preserved. That last approach is what Releo is building.
Frequently asked questions
How long does an API migration take?
It depends far more on the number of call sites and teams involved than on the size of the change. Small, isolated changes take hours; changes that touch many repositories routinely take weeks, mostly spent on finding call sites and verifying behavior.
How do I test an API migration?
Beyond unit tests, compare real responses from the old and new API versions for the same requests, and roll out gradually behind a flag while monitoring errors and business metrics.