My ESXi hosts used to have licensed essentials plus version 6 on them. I downloaded what I thought was version 6.5 but later discovered it is licensed as "Hypervisor". Apart from the license, the systems seem to be working fine. Should I just leave it installed and use my current version 6 key to update it or do I have to remove the Hypervisor and start all over again. I was told that the hypervisor install can be used as long as the key is licensed. Is that true?
Your 6.x license can be used for 6.5 licensing. In regards to Hypervisor vs Essentials Plus, it depends on what functionality you are utilizing. Hypervisor is less feature rich then Essentials Plus, namely, vCenter Compatibility, vMotion, Update Manager, Support features, etc. Here is a link to a wiki that has more detail. If I were you, I would assign you Essentials Plus license to your hosts. You can do assign the E+ license by simply adding the license and applying it to the host.