When you create your new vCenter, you are probably going to create it on the new 5.1 hosts in a cluster. If you then want to make a cluster-level change, you will be affecting the new vCenter VM, which you need to make the change in the first place. In the case of enabling EVC, we found that you can't do this with powered-on VMs.
To workaround it, we made a temporary cluster for vcenter to live in while we enabled EVC on the other cluster.
The temp cluster does not go in your old environment at all. This is just for your new environment.