<?php

defined( 'ABSPATH' ) or exit;

/** @ignore */
function _mc4wp_400_replace_comma_with_pipe( $matches ) {
    $old = $matches[1];
    $new = str_replace( ',', '|', $old );
    return str_replace( $old, $new, $matches[0] );
}

// get all forms
$posts = get_posts( array( 'post_type' => 'mc4wp-form', 'numberposts' => -1 ) );

foreach( $posts as $post ) {

    // find hidden field values in form and pass through replace function
    $old = $post->post_content;
    $new = preg_replace_callback( '/type="hidden" .* value="(.*)"/i', '_mc4wp_400_replace_comma_with_pipe', $old );

    // update post if we replaced something
    if( $new != $old ) {
        $post->post_content = $new;
        wp_update_post( $post );
    }
}