Hey,
HP image would contain additional drivers etc that might not be included with the VMware image. So that should help. As to the Virtual Machines, they can be migrated off to the other host and the hosta upgraded one at a time.
Are you doing the upgrade using update manager or booting off the ISO? If booting off the ISO, the only thing that needs to be done is to remember to choose the option of keeping your existing VMFS volumes intact.
Update failures using update manager usually rolls back the upgrade and thus minimises the chances of a failed upgrade.
Update manager documentation can be found at
Regards
a