The problem

When attempt to change category name or slug, the category ID is gone and replaced with a new number, which is the ID of an existing tag. The worst thing is that the old misplaced category record completely disappeared from both the blog and the Categories editor page.

This problem happens even when changing only category hierarchy (super & sub category relationship). The replacement appear to be random. It doesn't replace with a Tag that has exactly the same slug. In fact, the category record is unchanged if the slug is the same as another category's lug or another tag's slug,

Example 1

Example 2

Safe Method to Modify WPMU Category

I don't have a really good solution yet. At this time I've been hacking into the database to modify the slug value. You can follow the same steps too::

1. Open phpMyAdmin for your WordPress MU Database
2. Browse table wp_#_terms (# is the number of the blog being modified)
3. Search existing term record that have slug the same as the slug value you want to give to that category. Modify that existing slug to something else, because it's a slug of a tag (not as important as your category).
4. Search for the category you want to modify. Change the values to what you want.