Comment 2 for bug 2068671

Bugs which prevent us from getting to a new version are quite critical. We won't be able to directly change 3.4.2.
If we had to, we have things like `juju upgrade-controller --ignore-agent-versions`. That probably only skips the check for Juju agent versions, and not the application bases.

If we had to work around, we certainly can do the manual steps (ensuring that `agent-version` is correct in the controller model's config, which could easily trigger the UpgradeWorker to notice and do the upgrade. And if that was failing, down to the point of actually putting the agent binary on disk and updating the symlinks.

I think the config would be sufficient, though if you just edited the Mongodb record, you'd then probably need to restart the jujud agents because they wouldn't have the event that said the config has changed to then notice that they need to restart the workers themselves.

Certainly the check for "what can be deployed" needs to align with the check for "when can we upgrade".