rollback may replace instances, but sometimes it's not needed.

[Ethan Lynn]

1. Create a server with flavor_larger
2. update this server to flavor_smaller
3. update will fail and start to rollback
4. After rollback complete, instance will be recreated.

If resize a instance with flavor smaller, nova will through an error and refuse to resize this instance, so the instance is still in status active, and nothing changed.

But heat catch that error and start to rollback, during rollback ,instance will be replace by a new one. But actually it's no need to create a new instance and kill the old one, just reverse the template is ok.

[Ethan Lynn (ethanlynn)]

I think this bug may wait for convergence ready and then try to fix it.