SELECT
ChangedDateTime,
ROW_NUMBER() OVER(PARTITION BY ProductCode, ChangedDateTime ORDER BY ChangedDateTime DESC) change_id,
ProductCode
FROM `dataset.table`

Row ChangedDateTime change_id   ProductCode
1   2019-06-06 08:08:01.510 UTC 1   A11
2   2019-06-05 08:08:01.510 UTC 2   A11
3   2019-06-04 16:02:13.087 UTC 1   A11
4   2019-06-05 08:08:01.510 UTC 1   A110
5   2019-06-04 16:02:13.087 UTC 1   A110
6   2019-06-04 14:36:54.930 UTC 1   A110

Row ChangedDateTime change_id   ProductCode
1   2019-06-06 08:08:01.510 UTC 1   A11
2   2019-06-05 08:08:01.510 UTC 3   A11
3   2019-06-04 16:02:13.087 UTC 2   A11
4   2019-06-05 08:08:01.510 UTC 1   A110
5   2019-06-04 16:02:13.087 UTC 2   A110
6   2019-06-04 14:36:54.930 UTC 2   A110

SELECT
ChangedDateTime,
ROW_NUMBER() OVER(PARTITION BY ProductCode ORDER BY ChangedDateTime DESC) change_id,
ProductCode
FROM `dataset.table`