Since WordPress uses difrerent templates for both logged and not logged-in users, I ended up having to use some other hooks other than the default filter.

changing-form-fields-after-user-log-in.php

<?php
function alter_comment_form_fields($fields){

    $commenter = wp_get_current_commenter();

    $fields['author'] = '<p class="comment-form-input comment-form-author"><label for="author">' . __( 'This is a fancy label', 'your-text-domain' ) . '</label><input id="author" required name="author" type="text" placeholder="Seu nome" value="' . esc_attr( $commenter['comment_author'] ) . '" size="30" aria-required="true" /></p>';

    return $fields;
}

function logged_in_fields() {
    echo implode('', alter_comment_form_fields(array()));
}

add_filter('comment_form_default_fields','alter_comment_form_fields');
add_action( 'comment_form_logged_in_after', 'logged_in_fields' );
add_action( 'comment_form_after_fields', 'logged_in_fields' ); // this one will do the same thing as the filter comment_form_default_fields; chose one