Some questions for clarification:
- do all the VMs reside on shared storage, i.e. nothing on the host's local storage?
- did you already check the HCL (http://www.vmware.com/go/hcl) to ensure the hosts as well as the shared storage are both supported by the new version?
- do you use any vCenter Server related configuration (e.g. Virtual Distributed Switches)?
- do you use VM based backup software?
André
