Upgrade level -> Pay the difference

Solved. Added a check in functions.php so that if the select level is more expensive I do the difference.
Hope can be usefull.

function check_upgrade_level($level)
{
    global $wpdb,  $current_user;
    $levels = $wpdb->get_results("SELECT id,billing_amount FROM $wpdb->pmpro_membership_levels WHERE 1" );
    foreach ($levels as $levelDet){
        $levelDets[$levelDet->id]=$levelDet->billing_amount;
    }

    if($levelDets[$level->id] > $levelDets[$current_user->data->membership_level->id]){
        $diff = $levelDets[$level->id] - $levelDets[$current_user->data->membership_level->id];
        $level->initial_payment =  number_format((float)$diff,2);
        $level->billing_amount =  $level->initial_payment;
    }
    return $level;
}
add_filter("pmpro_checkout_level", "check_upgrade_level");

Thanks!