Change User Role on Fluent Forms Submission

fluent_form_user_role_change.php

<?php

 add_action('fluentform_submission_inserted', function ($insertId, $formData, $form) {
    if($form->id != 23) { // 23 is your target form id
        return;
    }

    $userId = get_current_user_id();
    if(!$userId) {
        return;
    }

    $user = get_user_by('ID', $userId);

    $roles = (array) $user->roles;

    $newUserRole = 'subscriber'; // Add your role name here
    if(in_array($newUserRole, $roles)) {
        return; // role already attached
    }

    $user->add_role( $newUserRole ); // new role attached to the user

}, 10, 3);