Recently we migrated some of the sub domain sites to BizHat.com. One of them was lyrics.bizhat.com. In old site we used movie names as category. In Bizhat we are using Lyrics as main category and moving all posts to this. In this process we used WordPress bulk edit option, this can be done quickly from the Posts menu.
Go to Posts > All Posts and then click on the checkbox at the top to select all of your posts.
After clicking the Apply button, you will see a new section appear on the page that includes bulk editing options
Here you can add the tags and select the new category. Once you select the category or add tags, click on the update button. This page will reload and all selected posts will be moved to the new category.