Am planning to decommission ESXi 5 and 5.5 hosts (License: VM Essential Kit)
Can i just decommission or must I deactivate license first?
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Hello.
One detail, you should keep (written) all your licenses for any eventuality.
If your company registered these licenses with VMware, you might be entitled to upgrade them to version 6.0 (if you had a subscription and support contract in place when version 6 was released).
VMware vSphere licenses are perpetual.