The Exchange team, or more specifically Jeff Mealiffe, has recently published a blog post detailing the sizing guidance for Exchange 2016. The short version is there is very little changed in the sizing guidance from Exchange 2013, but let’s take a closer look just to make sure we’re all on the same page.
The most obvious change in Exchange 2016 is that the installer no longer allows you to install the Client Access role separately from the Mailbox role. Those two roles still exist inside Exchange 2016, and everything functions pretty much the same, but you don’t have the option to install those roles on separate servers with Exchange 2016. As this is in-line with Microsoft’s recommended deployment methodology, this should not be much of a change for your Exchange environment. That being said, most of the Exchange environments I see still separate Client Access and Mailbox roles for some reason. If your migrating from Exchange 2013 to Exchange 2016, it will be important to go through the sizing calculator if you are going from separate CA servers to multi-role servers.
If you are upgrading from Exchange 2013 to Exchange 2016 with your Exchange 2013 servers already deployed as multi-role, then it is completely possible that you can just reuse the same hardware. Of course it is still important to re-do the sizing calculator to make sure as the CPU requirements for Exchange 2016 are a little higher than Exchange 2013, but if you sized your 2013 environment with a little CPU space to spare the same servers should work just fine.
For a customer with 4 Exchange 2013 servers deployed as multi-role, I imagine the upgrade process could go like this
Uninstall Exchange 2013 from two servers
Repurpose those two servers as Exchange 2016
Add two new Exchange 2016 servers into load balancer with 2013 servers
Migrate all mailboxes from 2013 DAG to 2016 DAG
Uninstall 2013 from two remaining servers
Install 2016 on two remaining servers
Accept the adulation of your peers and bosses for a smooth migration
Of course the entire migration process might be slightly more involved than that, but that list is close enough for government work, right? As long as you do the proper planning work, I think that is a reasonable high level migration plan for many organizations running Exchange 2013.
That planning work should, of course, include a sizing exercise with the most recent version of the sizing calculator.