AWS: editing a security group used in autoscaling group

When you edit an AWS security group, the changes are automatically applied to any EC2 instances associated with the security group.

When you change the launch configuration for your Auto Scaling group, any new instances are launched using the new configuration parameters, but existing instances are not affected.

So, my question is what happens when you edit the security group used by your Auto Scaling launch config? Are the changes applied only to new instances? Or do existing instances get the new security group rules as well?

Thanks!