We use Cloudformation to define everything in our stack. The AutoScalingGroup resource has an attribute called UpdatePolicy that describes how instances with th

We use Cloudformation to define everything in our stack. The AutoScalingGroup resource has an attribute called UpdatePolicy that describes how instances with the new AMI are deployed into the existing autoscaling group. As long as there is enough difference between the current number of instances in service and the maximum size of the ASG, existing instances won't be terminated until the new instances are passing healthchecks and the load balancer is sending them traffic. If the maximum has been reached, enough existing instances will be killed to provide room for the next batch of new instances.