Just write ordinary migrations with updates and such. I think you should able to manipulate the JSON without too much bother. Here's the functions available for JSON: http://www.postgresql.org/docs/9.4/static/functions-json.htm...

Here's more about the JSON type in Postgres: http://www.postgresql.org/docs/9.4/static/datatype-json.html

There's also Postgres' hstore, which I believe now shares the same backing as JSON(B), but may have a different API.