Finally I have successfully moved many posts from one category to another. The trick is to use this wonder “plugin” from Stephanie Booth.
This code essentially create an additional page in your control panel to allow you to add multiple posts into one category, remove multiple posts from one category, edit categories for multiple posts at the same time etc.
It’s truly wonderful! This is what you need to do:
1) Download the zipped file from here
2) Unzip the file, and upload the following files to the right folders where your WordPress blog resides:
batch-access.php goes in the plugins directory
batch-categories.php goes in the wp-admin directory
3) Go to your WP Panel > Plugins, activate “Batch Categories”
4) Once activated, you can click on the hyperlink to access your “batch-categories.php” page. Note that if you install your blog anywhere else other than your main folder, this hyperlink would not work. Your best bet is to access the page directly:
http://www.yourdomain.com/../wp-admin/batch-categories.php
with the “../” as whatever subfolders your blog resides in on your server
5) The “batch-categories.php” page is self explanatory. Play with it. However, you might need to note point (6) and (7), which I encountered
6) If you see nothing but a blank page when you try to load “batch-categories.php”, you need to edit this PHP file. Add in this line of code [ require_once(‘admin.php’); ] above this existing code line [ require_once(‘admin-header.php’); ]. Upload this edited file to overwrite the old one, and refresh that page. It should work now.
7) By default, “batch-categories.php” will only show 10 posts per page. If you want to show and edit more posts in one go, look for this line code in the PHP file [ $showposts=10; ]. Replace the number “10” with whatever number of posts you want, but be sensible and don’t hang your server!
Phew! Well, this is a documentation of my experience, and might not work for you. I am not a techie guy, so if you need support, please visit the following links:
Stephanie Booth’s Batch Categories Page
WordPress.org Support Forum
Useful information on GaryPaulson.net