I would like to upgrade my ESXi from 5.5 to 6.5 but my subscription and support has ended.
Depending on when your subscription has ended, you may be eligible to use the new version. If your subscription was active at the time when vSphere 6.5 was released, you should be able to upgrade your license keys in your My VMware portal.
I wasn't sure that I could downgrade from Standard to Essential
The binaries for all editions are the same, so there's no need to reinstall. However, in order to use/apply a license with less features, you need to make sure that no features are used, which are not covered by the new license.
André
