🔗 How to change a field type in an Ecto embedded schema with a migration

The other day, I had to change the type of a field in an Ecto embedded schema from a string to a list of strings. Sounds simple, right? Well, not quite—because this field already had existing data stored in PostgreSQL as JSONB. That meant I had to not only update my Elixir codebase but also write a migration to transform the stored data correctly.

Since this is a challenge others will likely encounter, I wanted to document the process, including the reasoning behind each step. If you ever need to change a field type in an embedded schema, this guide will help you do it safely without breaking your existing data.