Try this code, please note how I took the data you provided and included it in a way that can be used by others as well:

[font="Courier New"]-- 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

       [/font]

This works for the data you provided, but I can't guarantee it won't need to be changed to work for the real data.