Simple script to update/reassign Wordpress posts from old categories to new categories.

update_cats.php

<?php
/**
 * Simple script to update/reassign posts from old categories to new categories.
 * Creates any new categories that don't yet exist. 
 * Removes old categories that have now been emptied. 
 *
 * @author  Michael Niles <blindmikey@gmail.com>
 * @version 1.0.0
 */

// rudimentary security
// requires script must be loaded by site.com/update_cats.php?8765trfvb=54rhi9s4mmd1
if ( !isset($_GET['8765trfvb']) || '54rhi9s4mmd1' != $_GET['8765trfvb'] ) {
	die();
}

// Load WP instance
require_once('wp-config.php');
require_once('wp-load.php');
global $wpdb;

// Set vars
$limit          = -1; // -1 = does all
$cats_to_change = array(
	"Old Category Name" => "New Category Name", // Put your reassignments here
);

$count = 0;
foreach ( $cats_to_change as $old_cat => $new_cat )
{
    // If we're past our limit then stop
	$count++;
	if ( $limit < $count && 0 <= $limit )
	{
		echo __LINE__.': Reassignment limit reached. Stopping.<br/>'.PHP_EOL;
		break; 
	}

	$old_cat_id = get_cat_ID( $old_cat );
	$new_cat_id = get_cat_ID( $new_cat );

	// If the old cat is not found, skip
	if ( ! $old_cat_id )
	{
		echo __LINE__.': Old category NOT found: "'.$old_cat.'"<br/>'.PHP_EOL;
		echo __LINE__.': Skipping Reassignment for: "'.$old_cat.'"<br/>'.PHP_EOL;
		continue;
	}

	// If the new cat is the same as the old cat, skip
	if ( $new_cat_id == $old_cat_id )
	{
		echo __LINE__.': New category already exists as Old category: "'.$new_cat.'" = "'.$old_cat.'"<br/>'.PHP_EOL;
		echo __LINE__.': Skipping Reassignment for: "'.$old_cat.'"<br/>'.PHP_EOL;
		continue;
	}

	// Create New Category
	if ( ! $new_cat_id )
	{
		echo __LINE__.': New category NOT found: "'.$new_cat.'"<br/>'.PHP_EOL;
		echo __LINE__.': Creating category for: "'.$new_cat.'"<br/>'.PHP_EOL;

		$inserted_term = wp_create_category( $new_cat );

		if ( 0 == $inserted_term )
		{
			echo __LINE__.': Failure. Could NOT create new category. Skipping.<br/>'.PHP_EOL;
			continue;
		}
		else
		{
			$new_cat_id = $inserted_term;
			echo __LINE__.': Success. Created as cat id: "'.$new_cat_id.'"<br/>'.PHP_EOL;
		}
	}
	else {
		echo __LINE__.': New category already exists as: "'.$new_cat.'"<br/>'.PHP_EOL;
	}

	/**
	 * Get Posts by Old Category
	 */
	$posts = get_posts(array(
		'posts_per_page' => -1,
		'category' => $old_cat_id
	));

	foreach ( $posts as $post )
	{
		$post_id   = $post->ID;
		$cat_ids   = wp_get_post_categories( $post_id );
		echo __LINE__.': Reassigning categories for post ID: '.$post_id.'<br/>'.PHP_EOL;

		// remove old category
		if(($key = array_search($old_cat_id, $cat_ids)) !== false) {
			echo __LINE__.': Removing old category: "'.$old_cat.'" / ID: '.$old_cat_id.'<br/>'.PHP_EOL;
			unset($cat_ids[$key]);
		}

		// add our new category 
		echo __LINE__.': Adding new category: "'.$new_cat.'" / ID: '.$new_cat_id.'<br/>'.PHP_EOL;
		$cat_ids[] = $new_cat_id;

		// prepare our cat_ids
		$cat_ids = array_map( 'intval', $cat_ids );
		$cat_ids = array_unique( $cat_ids );

		// set the post to use only these cat_ids
		wp_set_object_terms( $post_id, $cat_ids, 'category' );
	}

	// we're done with the old - remove it. 
	echo __LINE__.': Deleting old category ID: '.$old_cat_id.'<br/>'.PHP_EOL;
	wp_delete_category( $old_cat_id );
}