code: private function upgrade1010($newConfigPaths)
      {
          $oldData = $this->scopeConfig->getValue($newConfigPaths['geoip_country_mapping']);
          try {
              $oldData = unserialize($oldData); // @codingStandardsIgnoreLine - used for conversion of old Magento format to json_decode
          } catch (\Exception $e) {
              $oldData = [];
          }
          $oldData = (is_array($oldData)) ? $oldData : [];
          $newData = json_encode($oldData);
          if (false === $newData) {
              throw new \InvalidArgumentException('Unable to encode data.');
          }
          $this->configWriter->save($newConfigPaths['geoip_country_mapping'], $newData);
          $this->cacheManager->clean([\Magento\Framework\App\Cache\Type\Config::TYPE_IDENTIFIER]);
      }
docs: Convert serialized data to magento supported serialized data
  @param $newConfigPaths