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The binaries are same for both free and paid version of ESXi.
You can add the license to the exisitng host, Install vCenter server and add ESXi host vCenter..
Thanks Aravind.
I have one more qus. Once removed Esxi hypervisor free license and add Esxi Essential plus license, after adding vcenter, Can I access and taking backup using Vmware Data recovery? Is it possible or need to install any addtionla packages on ESXi Host.
You have to deploy the VDR applicance and perform the necessary configuration to start taking the backup
Check the documentation for more information
Probably stupid question, but:
It's the same process to upgrade ESXi 5.1 from free to essentials ??
Yes adding the license key will unlock the feature relevant to Essentails license
Thank you very much for your fast reply!