ESX upgrade from 4.1.0 to 5.5 on a poweredge R710 ?

Hi

I have on site 3 dell pow r710 running esx 4.1.0.

they are in a "little cluster" of my vcenter : 3 hosts maximum with essential plus license.

if we upgrade our 3 esx to vmware 5.5, what is needed to do with the VM managing the vcenter ?

thanks