What's changed in Exchange Server 2016 Architecture

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* Client Access server (CAS) role has been removed in Exchange 2016 Architecture and added the client access services to the Mailbox role.

* Coexistence with Exchange Server 2013 - When you introduce Exchange Server 2016, you do not need to move the namespace. That’s right, the Exchange Server 2013 Client Access infrastructure can proxy the mailbox requests to the Exchange 2016 servers hosting the active database copy!. This provides flexibility to decide when move the namespace over to the new version. And not only that, now we can even have load balancer pools contain a mix of Exchange Server 2013 and Exchange Server 2016 servers. This means we can do a one-for-one swap – as we add Exchange 2016 servers, we remove Exchange 2013 servers. The following two diagrams reflects the Coexistence scenarios with Exchange 2013;

 

In Exchange Server 2016, there is a single building block that provides the client access services and the high availability architecture necessary for any enterprise messaging environment.

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For more details: https://blogs.technet.com/b/exchange/archive/2015/05/05/exchange-server-2016-architecture.aspx