How can I fix duplicated imported category data?

[icecold]

I was exactly doing that browsing manually and executing

UPDATE categories
SET categories_id=cat_Iwant
WHERE categories_id=cat_Iwant_to_remove
OR categories_id=...// (as many categories ID I want to copy ... this is working for subcats)

For instance if I want to remove 3 instances of Blue Rays that have different categories IDs say 456, 1200, 1300 and keep just 456 as categories ID, I do that ...

This last post is wrong If I do that I will get Duplicate keys MySQL errors the correct way is to go for parent_id instead of category id, and when there are no more subcats and it contains products I do the same but in "products_categories" table instead.