Postgres 13

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CREATE TABLE t (id INT, jsonb_t JSONB);
INSERT INTO t VALUES
(2,'[ {"x" : 1 , "y": 3} , {"x" : 3 , "y": 3} , {"x": 8, "y" : 2} ]'),
(3,'[ {"x" : 1 , "y": 4} , {"x" : 4 , "y": 3} , {"x": 5, "y" : 9} ]')
2 rows affected

SELECT jsonb_agg(jsonb_build_object('x', x, 'y', y))
  FROM (SELECT CASE
                 WHEN row_number() over() = jsonb_array_length(jsonb_t) THEN
                  x + 1
                 ELSE
                  x
               END AS x,
               CASE
                 WHEN row_number() over() = jsonb_array_length(jsonb_t) THEN
                  y - 1
                 ELSE
                  y
               END AS y
          FROM t, jsonb_to_recordset(jsonb_t) AS(x INT, y INT)
         WHERE id = 3) AS j


jsonb_agg


[{"x": 1, "y": 4}, {"x": 4, "y": 3}, {"x": 6, "y": 8}]