-- test table to hold test data
DECLARE @products TABLE(flag bit, product_id INT, product_id_group INT, id INT)

-- insert test data
INSERT INTO @products
   SELECT      
       0,             123456,                NULL,        456
   UNION ALL
   SELECT
       0,             789012,                NULL,        456
   UNION ALL
   SELECT
       1,             789012,                NULL,        456
   UNION ALL
   SELECT
       0,               1,                       456789,    132    
   UNION ALL
   SELECT
       0,               1,                       156894,    132
   UNION ALL
   SELECT
       1,               1,                       456789,    132

-- proposed solution
;WITH cteCounts AS
   (
   SELECT
       COUNT(DISTINCT flag) AS flag_count,
       ISNULL(product_id_group, product_id) AS product_id,
       id
   FROM
       @products 
   GROUP BY
       ISNULL(product_id_group, product_id) ,
       id
   )
   SELECT
       A.id,
       A.product_id AS new_product,
       B.product_id AS old_product
   FROM
       cteCounts A JOIN
       cteCounts B ON
           A.id = B.id AND
           A.product_id <> B.product_id
   WHERE
       A.flag_count = 1