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' );
  }
 }
}