Upgrading from v2.x to v3 is fairly simple. Below are the major changes that are released as a part of CometChat Pro v3: Please follow the setup instructions to upgrade to the latest V3 version.
v3 apps are compatible only with v3.x SDKs and will not function with v2.x SDKs. In case you want to migrate an existing v2 app to v3, you can achieve this from the CometChat Dashboard.

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Delivery Receipts

In v2.x, the messages were being marked as delivered internally by the SDKs. Starting v3, we have stopped marking the messages as delivered internally and it will have to be managed via the SDK. You can read more about the markAsDelivered() method here.

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Read Receipts

Starting v3.0, the method signature has been changed. Now the markAsRead() takes 4 parameters: messageId,receiverId, receiverType & senderId as input. This method will mark all the messages before the provided messageId for the conversation with the specified receiverId as read. Also, we have added one more signature of markAsRead() which takes only 1 parameter: message You can read more about the markAsRead() method here. 
//v2.x
CometChat.markAsRead(messageId, receiverId, receiverType);

//v3.x
CometChat.markAsRead(messageId, receiverId, receiverType, senderId);
// OR
CometChat.markAsRead(message);

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User/Group Tags in Conversation

In v2.x, the user/group tags were by default fetched in a Conversation Object while fetching the Conversation List. Starting v3, we have added a method withUserAndGroupTags() to the ConversationsRequestBuilder class to fetch the user/group tags in a Conversation Object while fetching the Conversation List. 
ConversationsReqquest conversationsRequest = new ConversationsRequest.ConversationsRequestBuilder()
  .setLimit(10)
  .withUserAndGroupTags()
  .build();