Ask HN: How do you handle data migrations in SQL databases?

There was a recent discussion about handling schema migrations (https://news.ycombinator.com/item?id=19880334). Following up on this, if you deploy a schema migration, sometimes there's a data migration that needs to follow. For example, I recently wanted to "promote" a field from a JSON column in a postgres database to a SQL table column. It's easy enough to add the new column, but what tools and patterns are out there for handling the data migration?