Here is what I can think:
- Upgrade your 5.0 vCenter to 5.5. Or fresh install of vCenter if you do not care performance history/data etc.
- If you upgrade VC from 5.0 to 5.5, you host cluster will be as it is with your old servers (5.0). In case of fresh Vc install, you will have to create host cluster & other configuration again.
- For the sake of steps, I assume, you have done VC upgrade (5.0 -5.5 latest). Hence your host cluster is already there.
- Install ESXi 5.5 latest on new servers & mount the LUNs to these servers/hosts.
- Add these new servers to host cluster. Now your host cluster will have old hosts and new hosts.
- At this moment, all your LUNs are shared across old and new hosts/servers.
-Make sure vMotion network is configured properly across all hosts (new as well old): You have enterprise server, hence you can leverage vMotion.
-If all CPUs are compatible with vMotion in old & new hosts, then vMotion will be smooth. If CPUs are from same family but feature sets are different, you will have to configure EVC to do successful vMotion.
-Start migrating your old hosts VMs to New hosts (I mean, vMotion ie. without powering them down)
-Once the all VMs are migrated to new hosts, put your old hosts into maintenance mode & take all old host out of cluster.
- Remove from maintenance mode (old hosts) and remove these hosts from vCenter. Finally unmount LUNs from old hosts.
If you have installed fresh vCenter server, then you need to create new host cluster & add all (new + old ) hosts to new cluster. Rest of the steps are same.
Refer: EVC : VMware KB: Enhanced vMotion Compatibility (EVC) processor support
Note: I am assuming you have not enabled any cluster features on host cluster such as DRS/DPM/HA etc. If yes, you can disable during migration.
Let me know if you need any help/clarification