title: Default to existing value in Terraform
body: <p>Does anyone know if there's a way in terraform to default settings to that of an existing resource, and only set them when creating a new resource?</p>
  
  <p>For example when I'm creating a new instance, I'll want to use the latest AMI, but for an existing instance (which should have all the latest patches, etc), there'd be no need to update the AMI.</p>