If you are adding hosts with lower EVC mode to a cluster of higher EVC mode, your VMs will be running at this level until they are powered off and on again (note: a restart operation is not sufficient).
So if you add your 'Merom' hosts to your "Nehalem/Westmere" cluster, you will notice your VMs list "Merom" as their EVC mode- they just won't have access to the additional CPU features of the higher EVC mode.
The only time you have to have outages for migrations is if you are going to a lower EVC mode, as the CPU features the VMs are running are being reduced. Of course the only caveat to this is the power cycle requirement to go up a level, but your VMs will still run.