ESX 3 for ESXi 5.1 and Cluster HA

Hello,

I have a clientthat has two3ESXhostswith32GB ofRAM, both connectedto anEquallogicstorage, eachESX hosthas two1TBLUNwithmultiple virtual machines.

The customerwants tomigrate tovSphere5.1usingvCenterto enableDRSandHA.

Can I upgradedirectlyfromESXtoESXi53?

Or, the correctLUNwoulddetachallthehosts, format and installESXi5.1, reattachall theLUNto bothhostsso that they haveaccess to allVMSandonly afterinstallingvCenterandcreate thecluster?

Whatfriendssuggestme?

I need todo this safely, withoutrisk of corruptingmachinesvirtual consolesavailable.

Thank you all!