Hi,
In the category tree you can still click on the categories and then, in the url, you can find the category_id (via the parameter "filter_id".
Then, when you edit a category, you can see the parameter "cid", which contain the ID of the category.
So in the url, just replace that parameter with the ID of the category you want to edit, and you will be able to change his parent category.
The other solution will be to use a mass action and replace the "category_parent_id" for all categories without a "category_parent_id" (equal to 0) and where the category_id is greater than 2.
Regards,