Enable EVC on a Running Cluster

Hello,

I have a doubt on enable EVC on a existing and active cluster.

I have a Cluster with:

2 x Dell R320 with Intel Xeon X3430 @ 2.40 with 4 core without hyperthreating.

EVC now is disable.

This two Hosts have 6 virtual machine running on it, without DRS, only HA and Vmotion.

Now he have bought a new Dell R320 but with a Intel E5-2430 @ 2.20 with 6 cores and hyperthreating.

All hosts are Vsphere 5.0.0 update 1

I added the new host to the HA cluster without problem, but, when i try to migrate a VM it fail because of CPU generation compatibilty.

Now I want to enable EVC, but I do not know if i can enable EVC compatibily without stopping the VM on the cluster.

Is this correct? Can i do it in a hot way? Witch compatibilty EVC mode I have to configure?

Thanks a lot.