Hi
The fastest built in way I know to merge categories is this:
1) depending on how many on posts need to be merged, change the # of posts per page the posts page displays – set it high
– click Screen Options at the top right of the posts page and set the # per page to 100 or 200
2) use the category filter above the posts list on the posts page. filter for the category you want to eliminate so only those posts display on the posts screen.
3) Click the checkbox next to Title at the top of the left column of the post listing. This will check every post on the page
4) Set the Bulk Actions pulldown to Edit and click Apply
5) In the little window that opens up select the category you want to add to all of the selected posts, then click update. (You can’t remove a category on this bulk update screen, only add one).
6) Its OK that you can’t remove the category on the bulk edit screen because merging means you want to altogether remove the category you just selected from anyway. Go to the Category page, under the Posts section of the Dashboard menu, find the category you no longer want, and delete it.
You have now taken all posts in one category, added them to a different category, and deleted the original category.
