nicodelienne

Thanks for reply !
So if I understand, I must refer to the old field’s id to create the meta box.

Like this :

// 1st meta box
    $meta_boxes[] = array(
        'id'         => 'personal',
        ...
        'fields' => array(
            array(
                'name'  => __( 'Full name', 'textdomain' ),
                'id'    => $prefix . 'id_of_my_old_field',
                'type'  => 'text'
            ),
        )
    );

You’re right. I’m just looking for a solution and I hope that future versions of WordPress will consider this case…

I’m curious if anyone else has found a better solution than getting into the core…

I think I succeeded. I had already removed ‘blog ‘ in :
wp-includes/ms-functions.php
wp-admin/network/site-new.php
$illegal_names = array_merge($illegal_names, apply_filters( 'subdirectory_reserved_names', array( 'page', 'comments', 'blog', 'files', 'feed' ) ) );

Now I did the same thing in /wp-includes/ms-settings.php

$reserved_blognames = array( 'page', 'comments', 'blog', 'wp-admin', 'wp-includes', 'wp-content', 'files', 'feed' );

…and it seems to work !
No more 404 error, the posts are retrieved correctly…
I check if there are no other problems

your main blog STILL thinks that belongs to it

There should be a way to fix it! Many customers want a blog with this url. The multisite is useful to separate the site from the blog