Update order status after a specific product id is purchased

add_action( 'woocommerce_payment_complete', 'simon_payment_complete' );
function simon_payment_complete( $order_id ) {
	$order = wc_get_order( $order_id );
	$items = $order->get_items();

	$product_ids_to_check = [ 123, 345 ]; // product ids you want to check against

	foreach ( $items as $item ) {
		if ( in_array( $item->get_product_id(), $product_ids_to_check ) ) {
			$order->update_status( 'pending', 'order_note' ); // order note is optional, if you want to  add a note to order
            		break;
		}
	}
}