Changing a field from "one" to "many" sounds trivial until three different consumers depend on it: an admin studio, a GraphQL API used by web and mobile apps, and a public API used by customers' own integrations.

This talk walks through a real migration on an event technology platform, where a single-value relationship became a multi-value, ordered one. We will cover how we designed the new schema alongside the old one, how we kept existing queries working, how we backfilled data safely, and how we decided when a deprecated field could finally go.