How do you guys handle data migrations (not schema migrations)?

For context - we have a RDB that has a column for "params" which is basically a JSON representation of a report object in our application.

Problem arises whenever we want to add new functionality that requires an API change we need to "migrate" the data from one JSON representation to another (or just add another translation layer whenever we fetch the data from the DB).

It's becoming unsustainable and every new piece of functionality we add needs to account for time to do this data migration (or adding a translation layer).

To be specific - this isn't a schema migration - we're not adding, removing or changing any columns. It's a mutation of the data itself.