add_action( 'woocommerce_order_status_completed', 'schooliner_change_role_on_purchase' );

function schooliner_change_role_on_purchase( $order_id ) {

	$order = new WC_Order( $order_id );
	$items = $order->get_items();

	$product_id = your_product_id;

	foreach ( $items as $item ) {

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

            $user->remove_role( 'customer' ); 

            $user->add_role( 'your_role' );
		}
	}
}