According to the HCL, the Intel Xeon 56xx Series CPU's EVC Modes are Intel® Merom Generation, Intel® Penryn Generation, Intel® Nehalem Generation and Intel® Westmere Generation, where the latter is the one used without EVC enabled. Intel Xeon 55xx Series CPU's only support the first 3 Modes, which means you will have to run the hosts in an EVC enabled cluster. Since the VM's on the X56xx hosts use newer CPU features you may not be able to change the EVC mode without downtime.
André