Hello.
All operating systems are at a certain risk when patches are applied and / or a set of patches (Update, Service Pack etc.), VMware vSphere is not the exsection.
The general rule of an infrastructure is first testing the patches on the development servers (or less critical) and then if everything is fine (a prudential time is expected), apply them to those of production.
Occasionally, an issue or bug will require admins to revert to a previous build or version of ESXi. In the event you patch your hosts (as opposed to fresh installs), it is possible to rollback to a prior installed version via the GUI. Rollbacks should not be taken lightly. If you are reverting in a production environment, discuss options with support first.
Link:
http://www.virtubytes.com/2017/11/16/downgrade-esxi-6-5/
Enrique Espinel
Senior Technical Support on IBM, Lenovo, Veeam Backup and VMware vSphere.
VSP-SV, VTSP-SV, VTSP-HCI, VTSP
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