not-best-idea.php

<?php
/*
Plugin Name: Replace old shortcode
Description: This will remove the Blackbird Pie shortcode when it's found via a filter
Version: 0.2

From http://wordpress.org/support/topic/how-do-i-remove-only-a-part-of-a-shortcode

*/

add_filter( 'the_content' , 'mh_remove_shortcode' , 0 );

function mh_remove_shortcode( $content ) {
global $wpdb, $post;
// Regex to find all [blackbirdpie url="twitter"] shotcodes.
$mh_url_regex = "/\[blackbirdpie\ url\=\"(http|https)\:\/\/twitter\.com\/.*\"]/";

// If we get any hits then remove the the shortcode
if ( preg_match_all( $mh_url_regex , $content, $mh_matches ) ) {;
        // Go through that array and remove the shortcode
        for ( $mh_count = 0; $mh_count < count( $mh_matches[0] ); $mh_count++ )
                {
                $mh_with_shortcode = $mh_matches[0][$mh_count];
                $mh_without = str_replace( '[blackbirdpie url="' , '' , $mh_matches[0][$mh_count] );
                $mh_without = str_replace( '"]' , '' , $mh_without );

                $content = str_replace( $mh_with_shortcode  , $mh_without , $content );
                // This is probably all wrong...
                $wpdb->update( $wpdb->posts, array( 'post_content' => $content ),
                        array( 'ID' => $post->ID )
                        );
                }
                // Make the page reload
                add_filter( 'wp_head' , 'mh_insert_redirect' );
        }
return $content;
}

function mh_insert_redirect() {
global $post;
        echo '<meta http-equiv="refresh" content="0;url=' . get_permalink( $post->ID ) . '">';
}