Editing it in the DB is really easy using PhpMyAdmin. The table is modx_categories. As long as you don’t change anything but the names of the specific categories, it will do exactly what you want.
Here’s something to think about, though. In 0.9.7, the last time I looked, all resources are available in the tree at the left side of the screen and every category is duplicated under every entry. In other words, if you have a category called "my snippets," it will be duplicated under documents, chunks, and files, as well as under the snippets entry. Each category appears under every element. Most of my categories were just annoying in the new system and I ended up deleting most of them.
I don’t know if this system will still be there in the final version of 0.9.7 (I hope not -- it would be much more useful to me if categories were element-specific or you could specify which elements they belong to), but I’m not creating a lot of categories until I know for sure.
Bob
Most excellent!
Bob