Query help for splitting departments

We want to restructure our categories and I wonder if someone can help me with a query to make this a more efficient process. We want to split the existing department into department and category. For example we have Art Glass/Bohemian as department with some minor categories that we won't be using anymore, and we now want the department to be Art Glass and the Category to be Bohemian. We have over 50 departments set up like the one above with over 22000 items, so one by one is not an option. Any help please would be so greatly appreciated. Thank you! diana