Thank you for your response.
Let me clarify and please point out if I am not understanding:
- Upgrade vCenter Server - Take existing 4.1 U3 snapshot it just to be sure, then upgrade it directly from 4.1 to 5.5.
Because vCenter is backwards compatable, existing 4.1 hosts will still be managed in the time being.
- Add the two new Dell R620s with fresh ESXi 5.5 installs. Join them to the new cluster with newly updated vCenter 5.5.
- Vmotion vm's from existing R710's to new 620s to prevent downtime.
- Upgrade existing R710's ESX 4.1 to ESXi 5.5
- Vmotion vm's back and distribute them evenly across all servers
- Upgrade vmware tools on each VM as time permits to allow for guest OS reboots.
- Upgrade datastores as time permits (is any downtime required?)