Can I alter an HSTORE column to JSON datatype? Is there a way to modify an existing column of HSTORE datatype (that has data) to JSON datatype in a Postgres database?
You can simple use hstore_to_json function to get a json out of a hstore. To change the table you can use the ALTER TABLE command.

Example:

ALTER TABLE table_name
    ALTER your_column_name TYPE json USING hstore_to_json(your_column_name);


There is hstore_to_jsonb too if you want jsonb instead of json.