WordPress: Only allow one post in a category at a time

gistfile1.aw

<?php
/*
Plugin Name: Move Old Urgent Posts
*/

function es_update_urgent_post_cats( $post_id ) {
	// Get our category
	$the_category     = 'urgent';
	$the_category_obj = get_category_by_slug( $the_category );
	$the_category_id  = $the_category_obj->term_id;

	$new_category     = "$the_category-archive"; // This category must exist
	$new_category_obj = get_category_by_slug( $new_category );
	$new_category_id  = $new_category_obj->term_id;

	// If the post being published isn't in the category we care above, move on
	if ( ! has_category( $the_category, $post_id ) )
		return;

	// Get all posts in our category except the newest one
	$args = array(
		'category_name' => $the_category,
		'orderby'       => 'date',
		'order'         => 'DESC',
		'offset'        => 1,
	);
	$query = new WP_Query( $args );

	// Loop through them
	if ( $query->have_posts() ):
		while ( $query->have_posts() ):
			$query->the_post();

			// Move any urgent posts to the archive category
			$cats   = wp_get_post_categories( $post->ID );
			$cats   = array_diff( $cats, array( $the_category_id ) );
			$cats   = array_values( $cats );
			$cats[] = $new_category_id;

			wp_set_post_categories( $post->ID, $cats );

		endwhile; // have_posts()
	endif; // have_posts()

	// Cleanup after ourselves
	wp_reset_postdata();
}
add_action( 'post_publish', 'es_update_urgent_post_cats' );