Comment 3 for bug 1243768

Note that you can simulate this a little bit by doing multiple service deploys.

So you do something like:

 juju deploy cs:cassandra-10 cassandra-A
 juju add-unit -n 100 cassandra-A

Then when you want to upgrade
 juju deploy cs:cassandra-11 cassandra-B
 juju add-unit -n 10 cassandra-B
 juju remove-unit -n 10 cassandra-A

See that things keep working:
 juju add-unit -n 50 cassandra-B
 juju remove-unit -n 50 cassandra-A
etc

However, you'd need a way to configure the charm so that its configuration can cross services. (Peer relations only work inside a service ATM.)

The nice thing about this method is that it gives you very fine control over the process, and it lets you do the upgrade as clearly bringing up *more* units of work, rather than shutting down the units before adding more capacity. (though you can always remove-unit before you add-unit if you want that.)

It is a workaround, though, and being able to roll out upgrades in place would be very nice to have.