<?php
$usernames = ['username1', 'username2', ...]; // An array of the usernames of the users you want to change the role for
$new_role = 'new_role'; // The new role you want to assign to the users

foreach ($usernames as $username) {
    $user = get_user_by('login', $username);
    if (!empty($user)) {
        $user->set_role($new_role);
    }
}
?>