You can enable Enhanced vMotion Compatability (EVC) on an HA cluster to level the playing field CPU wise when you run a cluster of servers with mixed CPUs.
A back ground article - kb.vmware.com/kb/1005764
Generally you would create the cluster and move the hosts and VMs over to it (VMs would be powered off). You can try to enable it on an existing cluster but generally you get an error about powered on VMs. Ideally you would want to set the EVC level using one of the old hosts as a new server would support EVC levels that the older hosts would not.
So you could do something like this:
1) Create a new cluster and enable HA
2) Migrate the VMs from one host and move the host to the new cluster.
3) Enable EVC
4) Shutdown some VMs, migrate them to the new cluster and power them on.
5) Repeat until all VMs and hosts are in the new cluster.