Sindbad~EG File Manager

<?php

defined('ABSPATH') or exit;

// find all form posts
$posts = get_posts(
    [
        'post_type'   => 'mc4wp-form',
        'post_status' => 'publish',
        'numberposts' => -1,
    ]
);

// set form message texts
$message_keys = [
    'text_subscribed',
    'text_error',
    'text_invalid_email',
    'text_already_subscribed',
    'text_required_field_missing',
    'text_unsubscribed',
    'text_not_subscribed',
];

foreach ($posts as $post) {
    $settings = get_post_meta($post->ID, '_mc4wp_settings', true);

    foreach ($message_keys as $key) {
        if (empty($settings[ $key ])) {
            continue;
        }

        $message = $settings[ $key ];

        // move message setting over to post meta
        update_post_meta($post->ID, $key, $message);
        unset($settings[ $key ]);
    }

    // update post meta with unset message keys
    update_post_meta($post->ID, '_mc4wp_settings', $settings);
}