,
			'wpo_wcpdf_documents_settings_invoice' => array(
				'enabled'					=> 1,
				// 'attach_to_email_ids'		=> array(),
				// 'display_shipping_address'	=> '',
				// 'display_email'				=> '',
				// 'display_phone'				=> '',
				// 'display_date'				=> '',
				// 'display_number'			=> '',
				// 'number_format'				=> array(),
				// 'reset_number_yearly'		=> '',
				// 'my_account_buttons'		=> '',
				// 'invoice_number_column'		=> '',
				// 'invoice_date_column'		=> '',
				// 'disable_free'				=> '',
			),
			'wpo_wcpdf_documents_settings_packing-slip' => array(
				'enabled'					=> 1,
				// 'display_billing_address'	=> '',
				// 'display_email'				=> '',
				// 'display_phone'				=> '',
			),
			'wpo_wcpdf_settings_debug' => array(
				// 'legacy_mode'				=> '',
				// 'enable_debug'				=> '',
				// 'html_output'				=> '',
				// 'html_output'				=> '',
				'enable_cleanup'				=> 1,
				'cleanup_days'					=> 7,
			),
		);
		foreach ($settings_defaults as $option => $defaults) {
			add_option( $option, $defaults );
		}

		// set transient for wizard notification
		set_transient( 'wpo_wcpdf_new_install', 'yes', DAY_IN_SECONDS * 2 );

		// schedule the yearly reset number action
		if ( ! empty( WPO_WCPDF()->settings ) && is_callable( array( WPO_WCPDF()->settings, 'schedule_yearly_reset_numbers' ) ) ) {
			WPO_WCPDF()->settings->schedule_yearly_reset_numbers();
		}
	}

	/**
	 * Plugin upgrade method.  Perform any required upgrades here
	 *
	 * @param string $installed_version the currently installed ('old') version
	 */
	protected function upgrade( $installed_version ) {
		// only upgrade when php 5.6 or higher
		if ( version_compare( PHP_VERSION, '5.6', '<' ) ) {
			return;
		}

		// sync fonts on every upgrade!
		$tmp_base = WPO_WCPDF()->main->get_tmp_base();

		// get fonts folder path
		$font_path = WPO_WCPDF()->main->get_tmp_path( 'fonts' );

		// check if tmp folder exists => if not, initialize 
		if ( ! @is_dir( $tmp_base ) || ! wp_is_writable( $tmp_base ) || ! @is_dir( $font_path ) || ! wp_is_writable( $font_path ) ) {
			WPO_WCPDF()->main->init_tmp();
		} else {
			// don't try merging fonts with local when updating pre 2.0
			$pre_2 = ( $installed_version == 'versionless' || version_compare( $installed_version, '2.0-dev', '<' ) );
			$merge_with_local = !$pre_2;
			WPO_WCPDF()->main->copy_fonts( $font_path, $merge_with_local );
		}

        // to ensure fonts will be copied to the upload directory
        delete_transient( 'wpo_wcpdf_subfolder_fonts_has_files' );
		
		// 1.5.28 update: copy next invoice number to separate setting
		if ( $installed_version == 'versionless' || version_compare( $installed_version, '1.5.28', '<' ) ) {
			$template_settings = get_option( 'wpo_wcpdf_template_settings' );
			$next_invoice_number = isset($template_settings['next_invoice_number'])?$template_settings['next_invoice_number']:'';
			update_option( 'wpo_wcpdf_next_invoice_number', $next_invoice_number );
		}

		// 2.0-dev update: reorganize settings
		if ( $installed_version == 'versionless' || version_compare( $installed_version, '2.0-dev', '<' ) ) {
			$old_settings = array(
				'wpo_wcpdf_general_settings'	=> get_option( 'wpo_wcpdf_general_settings' ),
				'wpo_wcpdf_template_settings'	=> get_option( 'wpo_wcpdf_template_settings' ),
				'wpo_wcpdf_debug_settings'		=> get_option( 'wpo_wcpdf_debug_settings' ),
			);

			// combine invoice number formatting in array
			$old_settings['wpo_wcpdf_template_settings']['invoice_number_formatting'] = array();
			$format_option_keys = array('padding','suffix','prefix');
			foreach ($format_option_keys as $format_option_key) {
				if (isset($old_settings['wpo_wcpdf_template_settings']["invoice_number_formatting_{$format_option_key}"])) {
					$old_settings['wpo_wcpdf_template_settings']['invoice_number_formatting'][$format_option_key] = $old_settings['wpo_wcpdf_template_settings']["invoice_number_formatting_{$format_option_key}"];
				}
			}

			// convert abbreviated email_ids
			if (isset($old_settings['wpo_wcpdf_general_settings']['email_pdf'])) {
				foreach ($old_settings['wpo_wcpdf_general_settings']['email_pdf'] as $email_id => $value) {
					if ($email_id == 'completed' || $email_id == 'processing') {
						$old_settings['wpo_wcpdf_general_settings']['email_pdf']["customer_{$email_id}_order"] = $value;
						unset($old_settings['wpo_wcpdf_general_settings']['email_pdf'][$email_id]);
					}
				}
			}

			// Migrate template path
			// forward slash for consistency/compatibility
			$template_path = str_replace('\\','/', $old_settings['wpo_wcpdf_template_settings']['template_path']);
			// strip abspath (forward slashed) if included
			$template_path = str_replace( str_replace('\\','/', ABSPATH), '', $template_path );
			// strip pdf subfolder from templates path
			$template_path = str_replace( '/templates/pdf/', '/templates/', $template_path );
			$old_settings['wpo_wcpdf_template_settings']['template_path'] = $template_path;

			// map new settings to old
			$settings_map = array(
				'wpo_wcpdf_settings_general' => array(
					'download_display'			=> array( 'wpo_wcpdf_general_settings' => 'download_display' ),
					'template_path'				=> array( 'wpo_wcpdf_template_settings' => 'template_path' ),
					'currency_font'				=> array( 'wpo_wcpdf_template_settings' => 'currency_font' ),
					'paper_size'				=> array( 'wpo_wcpdf_template_settings' => 'paper_size' ),
					'header_logo'				=> array( 'wpo_wcpdf_template_settings' => 'header_logo' ),
					'shop_name'					=> array( 'wpo_wcpdf_template_settings' => 'shop_name' ),
					'shop_address'				=> array( 'wpo_wcpdf_template_settings' => 'shop_address' ),
					'footer'					=> array( 'wpo_wcpdf_template_settings' => 'footer' ),
					'extra_1'					=> array( 'wpo_wcpdf_template_settings' => 'extra_1' ),
					'extra_2'					=> array( 'wpo_wcpdf_template_settings' => 'extra_2' ),
					'extra_3'					=> array( 'wpo_wcpdf_template_settings' => 'extra_3' ),
				),
				'wpo_wcpdf_documents_settings_invoice' => array(
					'attach_to_email_ids'		=> array( 'wpo_wcpdf_general_settings' => 'email_pdf' ),
					'display_shipping_address'	=> array( 'wpo_wcpdf_template_settings' => 'invoice_shipping_address' ),
					'display_email'				=> array( 'wpo_wcpdf_template_settings' => 'invoice_email' ),
					'display_phone'				=> array( 'wpo_wcpdf_template_settings' => 'invoice_phone' ),
					'display_date'				=> array( 'wpo_wcpdf_template_settings' => 'display_date' ),
					'display_number'			=> array( 'wpo_wcpdf_template_settings' => 'display_number' ),
					'number_format'				=> array( 'wpo_wcpdf_template_settings' => 'invoice_number_formatting' ),
					'reset_number_yearly'		=> array( 'wpo_wcpdf_template_settings' => 'yearly_reset_invoice_number' ),
					'my_account_buttons'		=> array( 'wpo_wcpdf_general_settings' => 'my_account_buttons' ),
					'invoice_number_column'		=> array( 'wpo_wcpdf_general_settings' => 'invoice_number_column' ),
					'invoice_date_column'		=> array( 'wpo_wcpdf_general_settings' => 'invoice_date_column' ),
					'disable_free'				=> array( 'wpo_wcpdf_general_settings' => 'disable_free' ),
				),
				'wpo_wcpdf_documents_settings_packing-slip' => array(
					'display_billing_address'	=> array( 'wpo_wcpdf_template_settings' => 'packing_slip_billing_address' ),
					'display_email'				=> array( 'wpo_wcpdf_template_settings' => 'packing_slip_email' ),
					'display_phone'				=> array( 'wpo_wcpdf_template_settings' => 'packing_slip_phone' ),
				),
				'wpo_wcpdf_settings_debug' => array(
					'enable_debug'				=> array( 'wpo_wcpdf_debug_settings' => 'enable_debug' ),
					'html_output'				=> array( 'wpo_wcpdf_debug_settings' => 'html_output' ),
				),
			);
			
			// walk through map
			foreach ($settings_map as $new_option => $new_settings_keys) {
				${$new_option} = array();
				foreach ($new_settings_keys as $new_key => $old_setting ) {
					$old_key = reset($old_setting);
					$old_option = key($old_setting);
					if (!empty($old_settings[$old_option][$old_key])) {
						// turn translatable fields into array
						$translatable_fields = array('shop_name','shop_address','footer','extra_1','extra_2','extra_3');
						if (in_array($new_key, $translatable_fields)) {
							${$new_option}[$new_key] = array( 'default' => $old_settings[$old_option][$old_key] );
						} else {
							${$new_option}[$new_key] = $old_settings[$old_option][$old_key];
						}
					}
				}

				// auto enable invoice & packing slip
				$enabled = array( 'wpo_wcpdf_documents_settings_invoice', 'wpo_wcpdf_documents_settings_packing-slip' );
				if ( in_array( $new_option, $enabled ) ) {
					${$new_option}['enabled'] = 1;
				}

				// merge with existing settings
				${$new_option."_old"} = get_option( $new_option, ${$new_option} ); // second argument loads new as default in case the settings did not exist yet
				${$new_option} = (array) ${$new_option} + (array) ${$new_option."_old"}; // duplicate options take new options as default

				// store new option values
				update_option( $new_option, ${$new_option} );
			}
		}

		// 2.0-beta-2 update: copy next number to separate db store
		if ( version_compare( $installed_version, '2.0-beta-2', '<' ) ) {
			$next_number = get_option( 'wpo_wcpdf_next_invoice_number' );
			if ( ! empty( $next_number ) ) {
				$number_store = new \WPO\WC\PDF_Invoices\Documents\Sequential_Number_Store( 'invoice_number' );
				$number_store->set_next( (int) $next_number );
			}
			// we're not deleting this option yet to make downgrading possible
			// delete_option( 'wpo_wcpdf_next_invoice_number' ); // clean up after ourselves
		}

		// 2.1.9: set cleanup defaults
		if ( $installed_version == 'versionless' || version_compare( $installed_version, '2.1.9', '<' ) ) {
			$debug_settings = get_option( 'wpo_wcpdf_settings_debug', array() );
			$debug_settings['enable_cleanup'] = 1;
			$debug_settings['cleanup_days'] = 7;
			update_option( 'wpo_wcpdf_settings_debug', $debug_settings );
		}

		// 2.10.0-dev: migrate template path to template ID
		// 2.11.5: improvements to the migration procedure
		if ( version_compare( $installed_version, '2.11.5', '<' ) ) {
			if ( ! empty( WPO_WCPDF()->settings ) && is_callable( array( WPO_WCPDF()->settings, 'maybe_migrate_template_paths' ) ) ) {
				WPO_WCPDF()->settings->maybe_migrate_template_paths();
			}
		}

		// 2.11.2: remove the obsolete .dist font cache file and mustRead.html from local fonts folder
		if ( version_compare( $installed_version, '2.11.2', '<' ) ) {
			@unlink( trailingslashit( $font_path ) . 'dompdf_font_family_cache.dist.php' );
			@unlink( trailingslashit( $font_path ) . 'mustRead.html' );
		}

		// 2.12.2-dev-1: change 'date' database table default value to '1000-01-01 00:00:00'
		if ( version_compare( $installed_version, '2.12.2-dev-1', '<' ) ) {
			global $wpdb;
			$documents = WPO_WCPDF()->documents->get_documents( 'all' );
			foreach ( $documents as $document ) {
				$store_name = "{$document->slug}_number";
				$method     = WPO_WCPDF()->settings->get_sequential_number_store_method();
				$table_name = apply_filters( "wpo_wcpdf_number_store_table_name", "{$wpdb->prefix}wcpdf_{$store_name}", $store_name, $method );
				if ( $wpdb->get_var( "SHOW TABLES LIKE '{$table_name}'" ) != $table_name ) {
					continue;
				}
				if ( is_callable( array( $document, 'get_sequential_number_store' ) ) ) {
					$number_store = $document->get_sequential_number_store();
					if ( ! empty( $number_store ) ) {
						$column_name = 'date';
						$query       = $wpdb->query( "ALTER TABLE {$number_store->table_name} ALTER {$column_name} SET DEFAULT '1000-01-01 00:00:00'" );
						if ( $query ) {
							wcpdf_log_error(
								"Default value changed for 'date' column to '1000-01-01 00:00:00' on database table: {$number_store->table_name}",
								'info'
							);
						} else {
							wcpdf_log_error(
								"An error occurred! The default value for 'date' column couldn't be changed to '1000-01-01 00:00:00' on database table: {$number_store->table_name}",
								'critical'
							);
						}
					}
				}
			}
		}

		// 3.0.0-dev-1: remove saved option 'use_html5_parser'
		if ( version_compare( $installed_version, '3.0.0-dev-1', '<' ) ) {
			// removes 'HTML5 parser' setting value
			$debug_settings = get_option( 'wpo_wcpdf_settings_debug', array() );
			if ( ! empty( $debug_settings['use_html5_parser'] ) ) {
				unset( $debug_settings['use_html5_parser'] );
				update_option( 'wpo_wcpdf_settings_debug', $debug_settings );
			}
		}
		
		// 3.3.0-dev-1: schedule the yearly reset number action
		if ( version_compare( $installed_version, '3.3.0-dev-1', '<' ) ) {
			if ( ! empty( WPO_WCPDF()->settings ) && is_callable( array( WPO_WCPDF()->settings, 'schedule_yearly_reset_numbers' ) ) ) {
				WPO_WCPDF()->settings->schedule_yearly_reset_numbers();
			}
		}
		
		// 3.5.7-dev-1: migrate 'guest_access' setting to 'document_link_access_type'
		if ( version_compare( $installed_version, '3.5.7-dev-1', '<' ) ) {
			$debug_settings = get_option( 'wpo_wcpdf_settings_debug', array() );
			if ( ! empty( $debug_settings['guest_access'] ) ) {
				unset( $debug_settings['guest_access'] );
				$debug_settings['document_link_access_type'] = 'guest';
				update_option( 'wpo_wcpdf_settings_debug', $debug_settings );
			}
		}
		
		// 3.6.3-dev-1: check if 'legacy_mode' is enabled, and disable it
		if ( version_compare( $installed_version, '3.6.3-dev-1', '<' ) ) {
			$debug_settings = get_option( 'wpo_wcpdf_settings_debug', array() );
			$update         = false;
			
			if ( ! empty( $debug_settings['legacy_mode'] ) ) {
				unset( $debug_settings['legacy_mode'] );
				$update = true;
			}
			
			if ( ! empty( $debug_settings['legacy_textdomain'] ) ) {
				unset( $debug_settings['legacy_textdomain'] );
				$update = true;
			}
			
			if ( $update ) {
				update_option( 'wpo_wcpdf_settings_debug', $debug_settings );
			}
		}
		
	}

	/**
	 * Plugin downgrade method.  Perform any required downgrades here
	 * 
	 *
	 * @param string $installed_version the currently installed ('old') version (actually higher since this is a downgrade)
	 */
	protected function downgrade( $installed_version ) {
		// make sure fonts match with version: copy from plugin folder
		$tmp_base = WPO_WCPDF()->main->get_tmp_base();

		// make sure we have the fonts directory
		$font_path = WPO_WCPDF()->main->get_tmp_path( 'fonts' );

		// don't continue if we don't have an upload dir
		if ($tmp_base === false) {
			return false;
		}

		// check if tmp folder exists => if not, initialize 
		if ( ! @is_dir( $tmp_base ) || ! wp_is_writable( $tmp_base ) || ! @is_dir( $font_path ) || ! wp_is_writable( $font_path ) ) {
			WPO_WCPDF()->main->init_tmp();
		} else {
			WPO_WCPDF()->main->copy_fonts( $font_path );
		}
	}

}

endif; // class_exists