vSphere products distinguish between upgrades, which make major changes to the software, patches and updates, which make smaller changes to the software, and migrations, which make changes to the software platform.
VMware product versions are numbered with two digits, for example, vSphere 6.5. A release that changes either digit, for example, from 5.5 to 6.0, or from 6.0 to 6.5, involves major changes in the software, and requires an upgrade from the previous version. A release that makes a smaller change, requiring only a patch or update, is indicated by an update number, for example, vSphere 6.0 Update 1.
For information about upgrading vCenter Server installations, see Upgrading the vCenter Server Appliance and Platform Services Controller Appliance or Upgrading vCenter Server for Windows.
For information about patching or updating vCenter Server, see Patching and Updating vCenter Server 6.5 Deployments
When you upgrade an ESXi host, some host configuration information is preserved in the upgraded version, and the upgraded host, after rebooting, can join a vCenter Server instance that has been upgraded to the same level. Because updates and patches do not involve major changes to the software, host configuration is not affected. For more details, see Upgrade or Update a Host with Image Profiles
When you upgrade a vCenter Server for Windows instance and at the same time convert it to a vCenter Server Appliance instance, it is a migration.
For information about migrating a vCenter Server installation to an appliance, see Migrating vCenter Server for Windows to vCenter Server Appliance.