Postgres 12

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CREATE TABLE mytable (
    mytext_col text
);

INSERT INTO mytable VALUES
('{"a":1, "b":2}'),
('[1,2,3]');
2 rows affected

ALTER TABLE mytable ALTER COLUMN mytext_col TYPE jsonb USING mytext_col::jsonb;
SELECT * FROM mytable


mytext_col


{"a": 1, "b": 2}

[1, 2, 3]