add_action( 'woocommerce_order_status_completed', 'vap_change_role_on_purchase_gold' );

function vap_change_role_on_purchase_gold( $order_id ) {

// get order object and items
	$order = new WC_Order( $order_id );
	$items = $order->get_items();

	$product_id = 2355; // that's a specific product ID

	foreach ( $items as $item ) {

		if( $product_id == $item['product_id'] && $order->user_id ) {
			$user = new WP_User( $order->user_id );

			// Remove old role
            $user->remove_role( 'customer' ); 

            // Add new role
            $user->add_role( 'pmpro_role_3' );
		}
	}
}