Doing this hot or cold depends on the hardware CPU vs EVC mode selected and this is a cluster feature only for vCenter. I remember I originally had an older host that wasn't compatible with all the selection and had to use the Sandy Bridge EVC, which meant shutting down the servers in Maintenance Mode to do. However, when I implemented my new host recently I was able to add EVC hot using Nehalem in my case anyway since the new host CPU matched the others using i7's.
Regards, CSSMP