I imagine that you have the lowest current baseline for your Intel(R)Xeon(R) CPU E5-2620 processor.
What i do in this cases is create a new cluster with the new hosts and enable EVC with the baseline of the other cluster, then vMotion the VMs from the first cluster to the second one (Guessing that you have Shared storage between the hosts).
Then you can disable EVC on the first cluster and move the empty hosts to the second one, doing this, the "older hosts" will take the EVC baseline.
I would add the Hosts into the existing cluster and then enable EVC. It will only let you select the EVC mode for the lowest common denominator. You won't be able to take advantage of any new CPU sets until the VM has been shutdown and powered back on, but you can schedule the VM outages at your leisure after the fact.