/**
 * Only charge billing amount for renewals.
 */
function my_pmpro_checkout_level($level) {
    // Only continue for recurring levels.
    if( ! pmpro_isLevelRecurring( $level ) ) {
        return $level;
    }
    global $wpdb, $current_user;

    // Does this user have a membership record for this level within the past 3 months?
      $enddate = date('Y-m-d H:i:s', strtotime('-3 months', current_time('timestamp')));
      $sql = "SELECT COUNT(id) FROM {$wpdb->pmpro_memberships_users} WHERE user_id = {$current_user->ID} AND membership_id = {$level->id} AND enddate > '{$enddate}' LIMIT 1";
      $old_level = $wpdb->get_var($sql);
    if($old_level) {
        $level->initial_payment = $level->billing_amount;
    }
    $level->cycle_number = 0;
    $level->billing_amount = 0;
    $level->cycle_period = '';
    return $level;
}
add_action('pmpro_checkout_level', 'my_pmpro_checkout_level');