The vCenter Server Appliance (VCSA) upgrade from 6.0 to 6.5 is a slick, well managed, process. The installer deploys a new appliance, transfers all the settings from the old one (including IP address and identity) and then powers the old appliance VM off. This provides a neat roll-back option if something has gone wrong- you just power down the new appliance and turn the old one back on.
In upgrading my homelab I came across a small gotcha in this however. If you have your old VCSA VM set to auto power on when the host is then the next time you reboot that host (for example when you’ve done a Hypervisor upgrade) the old VCSA will turn on. You could now, like me, be in the situation where two VCSAs are running at the same time and sharing an IP. In a larger environment this is unlikely to happen- the host-based VM Startup/Shutdown feature is largely replaced by HA and DRS rules – but in a smaller setup this is a common practise to ensure that the vCenter comes up when the lab is switched on.
The solution is to switch the Startup behaviour to manual for the old VCSA, and just to make sure disable the virtual NIC on that old VM. Of course once you are happy with the upgrade the old Virtual Machine can be archived/deleted altogether.