For example I have override class `Magento\Store\Block\Switcher` to `Vendor\Module\Block\Switcher` So I needs to add custom method, constructor, and protected variable in upgraded class `Vendor\Module\Block\Switcher`. 

I have tried following code but it is not working 

Add Code Like Below in `/etc/frontend/di.xml` file.

    <?xml version="1.0"?>
       <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
          <preference for="Magento\Store\Block\Switcher" type="Vendor\Module\Block\Switcher\Index" />
       </config>
       
Add below code in my override class `Vendor\Module\Block\Switcher`

    <?php 
    namespace Vendor\Module\Block\Switcher;
    
    class Index extends \Magento\Framework\View\Element\Template
    {
    
      protected $httpContext; //new variable
    
      public function __construct(
        \Magento\Framework\View\Element\Template\Context $context,
        \Magento\Framework\Data\Helper\PostHelper $postDataHelper,
        \Magento\Framework\App\Http\Context $httpContext, // Inject New Class Just before Data array if you want to inject
        array $data = []
        ) {
    
          $this->_postDataHelper = $postDataHelper;
          $this->httpContext = $httpContext; //New object Get of Injected Class
          parent::__construct($context, $data);
        }
    
        /**
         * Code by devloper
         */
        public function isLoggedIn()
        {
            return (bool)$this->httpContext->getValue(\Magento\Customer\Model\Context::CONTEXT_AUTH);
        }
    
        /**     
         * Code by devloper
         */
        public function getBankUrl()
        {
            if (!$this->isLoggedIn()) {
                return '<li class="customer-login"><a href="javascript:void(0);">Bank sd</a></li>';
            } else {
                return '<li class=""><a href="'.$this->getUrl('rewards/referral').'">Bank</a></li>';        
            }
        }
    }
    
    ?>