If this is common please give me that list of vendors.
My new Proliants can hot plug memory, disk controllers, disk drives but definitely not the CPU (offline, yes). vSphere has the capability to hot add vCPUs and memory. As far as I know swapping out CPUs in a running system is still uncommon.
Most of their P series (AIX Power systems) can hotplug almost anything also (I've never hot-swapped CPUs on one, but I thought I saw an option in SMIT for that).
My new Proliants can hot plug memory, disk controllers, disk drives but definitely not the CPU (offline, yes). vSphere has the capability to hot add vCPUs and memory. As far as I know swapping out CPUs in a running system is still uncommon.