Change default order status | WordPress.org

I figured it out. Here’s the code I used for variable products. Will require additional modification to apply to simple products as well.

/**
 * Set pre order status when order has any pre order items
 *
 * @param string   $status      Payment status that should be set on payment complete.
 * @param int      $order_id    Order ID.
 * @param WC_Order $order       Order object.
 *
 * @return string
 */
function vh_sto_wc_set_pre_order_status_on_payment_complete( $status, $order_id, $order ) {
    // Make a check that new status is "processing" as we don't want to do anything for virtual orders.
    // If you have to perform check for virtual orders as well then you can add or check ('processing' === $status || 'completed' === $status).
    if ( 'processing' === $status && ( 'on-hold' === $order->status || 'pending' === $order->status || 'failed' === $order->status ) ) {
        // Define has pre order false initially.
        $has_pre_order = false;

        // Loop through order items.
        foreach ( $order->get_items() as $item_id => $item ) {
            // Get the product object from the order item.
            $product = $item->get_product();
            // Get the product id.
            $product_id = $product->get_id();
			$variation_id = $item->get_variation_id();

            // Write your logic to validate if product is a pre order product or not.
            // then store in $pre_order_checked as true/false.
            $pre_order_checked = get_post_meta( $variation_id, '_wpro_variable_is_preorder', true );

            if ( $pre_order_checked == 'yes' ) {
                // assuming you perform a true condition test in if statement
                // then set has pre order to true.
                $has_pre_order = true;

                // Break the loop as we got what we want.
                break;
            }
        }

        if ( $has_pre_order == 'yes' ) {
            $status = 'preorder';
        } else {
            // Autocomplete the processing order if the pre-order is unidentified.
            $status = 'completed';
        }
    }

    return $status;
}
add_action( 'woocommerce_payment_complete_order_status', 'vh_sto_wc_set_pre_order_status_on_payment_complete', 20, 3 );