providers/aws: support updating ASG launch config [GH-904]

builtin/providers/aws/resource_aws_autoscaling_group.go

@@ -29,7 +29,6 @@ func resourceAwsAutoscalingGroup() *schema.Resource {
 			"launch_configuration": &schema.Schema{
 				Type:     schema.TypeString,
 				Required: true,
-				ForceNew: true,
 			},
 
 			"desired_capacity": &schema.Schema{
@@ -214,6 +213,10 @@ func resourceAwsAutoscalingGroupUpdate(d *schema.ResourceData, meta interface{})
 		opts.SetDesiredCapacity = true
 	}
 
+	if d.HasChange("launch_configuration") {
+		opts.LaunchConfigurationName = d.Get("launch_configuration").(string)
+	}
+
 	if d.HasChange("min_size") {
 		opts.MinSize = d.Get("min_size").(int)
 		opts.SetMinSize = true