While merging two different blogs together or while cleaning up the unnecessary tags and categories from your WordPress blog what you need to do is merge two different categories or you can say moving a category to another category. This tutorial explains you how you can easily move your blog’s categories and tags without creating any 404 error.
Its easy, what you need to do is install and activate Bulk Move in your WordPress site. After activation visit its options page and their you will be seeing two dropdown select box. From the left dropdown select box choose the category whose posts you want to move and from the right dropdown select the category to which you want your posts to be moved. That’s all for it. Enjoy.
Alternatively, assuming you know term_taxonomy_id of the old and new categories you can use phpMyadmin with a SQL statement as shown:
UPDATE wp_term_relationships SET term_taxonomy_id = '10' WHERE term_taxonomy_id = '299'
- table name is wp_terms_relationships
- old category term_taxonomy_id is 299
- new category term_taxonomy_id is 10
But remember to backup your database before attempting this or using a plugin would be a better choice if you are new in PhpMyAdmin. Thanks to MichaelH for suggesting this trick.