update to aws_autoscaling_group (launch_configuration) forces new resource.

[lesaux]

Hello, I would expect terraform to update the autoscaling group when I change the launch_configuration entry, and not to destroy and rebuild it. This is a value I could update in the EC2 console (whereas you cannot edit a launchconfig). In my case this causes an outage of the webservice I am configuring. My current workaround is to add a new autoscaling group and new launch configuration, wait for them to be up, and then delete both the old autoscaling group and the old launch config.
If terraform was to update the autoscaling group and not force a new resource, I would still need to perform some action to spin up instances with the new launch config, and get rid of the old instances.

I can think of several use cases for this, and one good example is that want to update the AMI I am using in this autoscaling group. Maybe be this is not the recommended way to update an AMI in an autoscaling group with terraform? Am I missing something? Thanks.

Below the output of terraform which shows the -/+ instead of the ~

-/+ aws_autoscaling_group.web-servers-autoscaling-new
availability_zones.#: "3" => "3"
availability_zones.1305112097: "us-east-1b" => "us-east-1b"
availability_zones.2762590996: "us-east-1d" => "us-east-1d"
availability_zones.3569565595: "us-east-1a" => "us-east-1a"
default_cooldown: "300" => ""
desired_capacity: "1" => "1"
force_delete: "" => ""
health_check_grace_period: "30" => "30"
health_check_type: "ELB" => "ELB"
launch_configuration: "web-servers-v1" => "web-servers-v2" (forces new resource)
load_balancers.#: "1" => "1"
load_balancers.724870401: "www-example-com" => "www-example-com"
max_size: "5" => "5"
min_size: "1" => "1"
name: "web-servers-autoscaling-new" => "web-servers-autoscaling-new"
termination_policies.#: "0" => ""
vpc_zone_identifier.#: "3" => "3"
vpc_zone_identifier.1903260895: "subnet-aaaaaaaa" => "subnet-aaaaaaaa"
vpc_zone_identifier.3856991568: "subnet-bbbbbbbb" => "subnet-bbbbbbbb"
vpc_zone_identifier.4021483473: "subnet-ccccccccc" => "subnet-ccccccccc"

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