This would allow upgrades to the cluster without any kind of perturbation to the running cluster at all. This is not really a RollingUpgrade. AlanRobertson notes that there is no budget assigned for this feature, and classifies it as future work, that is beyond 2004. Clearly the HeartbeatProgram doesn't provide this feature. As of now, no one who uses the HeartbeatProgram has ever asked for this capability. Getting a working RollingUpgrade capability seems a higher priority.