Bulk convert any number of custom post meta keys for all posts in a specific WordPress post type. Change the $posttype, $old_prefix, $new_prefix and the $keys array at the top of the plugin file. Important: This plugin fires on the init hook, so only run it once (deactivate this plugin immediately after all custom post meta keys have been updated.

gpp-bulk-convert-post-meta-keys.php

<?php

/**
 * Plugin Name: GPP Bulk Convert Post Meta Once
 * Plugin URI: http://graphpaperpress.com
 * Description: Converts post meta from one key format to another
 * Author: Thad Allender
 * Author URI: http://graphpaperpress.com
 * Version: 1.0
 * Text Domain: gpp
 * Domain Path: languages
 */

function gpp_bulk_convert_post_meta_run_once(){
	
	// config
	// the post type you want to change post meta keys for
	$posttype = 'themes';
	// the old post meta key prefix
	$old_prefix = 'gpptmb_';
	// the new post meta key prefix
	$new_prefix = 'gpp_';

    $args = array(
        'numberposts' => -1,
        'post_type' => $posttype
    );

    $posts = get_posts( $args );

    foreach ( $posts as $p ) :

        $keys = NULL;
		
		// all keys you want to change, excluding their common prefix
        $keys = array(
            'download' => 'download-id',
            'description' => 'theme-description',
            'ft1' => 'feature-title-1',
            'ft2' => 'feature-title-2',
            'ft3' => 'feature-title-3',
            'ft4' => 'feature-title-4',
            'fd1' => 'feature-description-1',
            'fd2' => 'feature-description-2',
            'fd3' => 'feature-description-3',
            'fd4' => 'feature-description-4',
            'fi1' => 'feature-image-1',
            'fi2' => 'feature-image-2',
            'fi3' => 'feature-image-3',
            'fi4' => 'feature-image-4',
            'quote' => 'theme-quote',
            'quoteattr' => 'theme-quote-attribution',
            'quoteurl' => 'theme-quote-url',
            'additional' => 'theme-additional-info'
        );

        foreach ( $keys as $key ) : 

            $meta = get_post_meta( $p->ID, $old_prefix . $key, true );

            if ( $meta ) :

                update_post_meta( $p->ID, $new_prefix . $key, $meta );
                delete_post_meta( $p->ID, $old_prefix . $key );

            endif;

        endforeach;

        if ( isset( $p ) && is_array( $p ) ) :
            
            // save the post
            wp_update_post( $p->ID );

            // unset the post
            unset( $p->ID );
        
        endif;

    endforeach; 

}
// fires on the init hook by default, so only run this once!
add_action( 'init',  'gpp_bulk_convert_post_meta_run_once' );