php bin/magento setup:upgrade

namespace ScandesignsProductTypeSetup;

use MagentoFrameworkSetupUpgradeSchemaInterface;
use MagentoFrameworkSetupModuleContextInterface;
use MagentoFrameworkSetupSchemaSetupInterface;

class UpgradeSchema implements UpgradeSchemaInterface
{
    public function upgrade(SchemaSetupInterface $setup, ModuleContextInterface $context)
    {
        if (version_compare($context->getVersion(), '2.0.1') < 0) {
            $setup->startSetup();
            $setup->getConnection()->addColumn(
                $setup->getTable('sales_order_item'),
               'product_custom_type',
               [
                   'type' => MagentoFrameworkDBDdlTable::TYPE_STRING,
                   'length' => '256',
                   'nullable' => false,
                   'default' => '',
                   'comment' => 'product custom type'
               ]
           );
           $setup->endSetup();
        }
    }
}

<?php

namespace ScandesignsProductTypeSetup;

use MagentoFrameworkSetupUpgradeSchemaInterface;
use MagentoFrameworkSetupModuleContextInterface;
use MagentoFrameworkSetupSchemaSetupInterface;
use MagentoFrameworkDBDdlTable;

class UpgradeSchema implements UpgradeSchemaInterface
{
    public function upgrade(SchemaSetupInterface $setup, ModuleContextInterface $context)
    {
        $installer = $setup;
        $installer->startSetup();
        $connection = $installer->getConnection();

        if (version_compare($context->getVersion(), '2.0.1') < 0) {
            $connection->addColumn(
            $installer->getTable('sales_order_item'),
               'product_custom_type',
               [
                   'type' => MagentoFrameworkDBDdlTable::TYPE_TEXT,
                   'length' => 256,
                   'nullable' => false,
                   'default' => '',
                   'comment' => 'product custom type'
               ]
           );
        }
         $installer->endSetup();
    }
}