There is something called Up-version proxy by which your Exchange 2013 CAS can handle all the connections for Exchange 2016 mailbox servers.

By having this you have a flexibility to introduce Exchange 2016 servers configure DAG on them , Move the mailboxes and do not worry about the CAS URL’s , since exchange 2013 have the capability to route the connections to exchange 2016 mailboxes.

Later you can introduce the CAS services into your LB’s , point your CAS URL’s to Exchange 2016 services.Exchange 2013 can proxy requests to Exchange 2016 and Exchange 2013 and one more good thing is that  Exchange 2016 can exist in the same load balanced CAS array.

So the best approach would be:

1.Prepare your AD
2.Make sure that you install CU10 on your Exchange 2013 servers
3.Bring Exchange 2016 servers
4.Configure the URL’s
5.Configure the certificates
6.Move few pilot users and test the coexistence
7.Configure DAG
8.Start staged migration by moving sets of mailboxes
9.Point all your CAS URL’s to point to your exchange 2016 services
10.Install certificates on the Exchange 2016 CAS servers
This might help you for the certificates
http://social.technet.microsoft.com/wiki/contents/articles/31783.configure-ssl-certificate-in-exchange-server-2016.aspx

Also there is Exchange 2016 sizing calculator is out. Please use them to configure according to your environment.

http://blogs.technet.com/b/exchange/archive/2015/10/15/ask-the-perf-guy-sizing-exchange-2016-deployments.aspx

http://blogs.technet.com/b/exchange/archive/2015/10/15/exchange-server-role-requirements-calculator-update.aspx

Note : Exchange 2007 and earlier won’t be supported for coexistence, So make sure you do not have any legacy versions running on your environment.

It will be much easier for you if you have Exchange 2013 in your environment since the CAS services can still remain in exchange 2013 until your migration is completed.

But if you have only  Exchange 2010 in your setup then you will need to move all of your external URL’s to exchange 2016 and place your SSL certificates into the Exchange 2016 servers and then start the migration.

Thanks 

Sathish Veerapandian

MVP – Exchange Server