Comment 3 for bug 1738728

We downgraded my model and controller to 2.2.6. The model is easy, the controller is a bit harder:

For the model:

 - In a mongo shell on the controller:
   db.settings.update({"_id": "$MODEL_UUID:e"}, {$set: {"settings.agent-version": "2.2.6"}})

 - Restart all model machine agents.

 - Wait a few seconds and restart all model unit agents.

 - "juju status --format yaml | grep version" should now report 2.2.6 across the board.

For the controller:

 - In a mongo shell on the controller:
   db.settings.update({"_id": "$CONTROLLER_MODEL_UUID:e"}, {$set: {"settings.agent-version": "2.2.6"}})

 - In each controller machine's agent.conf, change upgradedToVersion to 2.2.6.

 - Restart all controller machine agents.

 - Restart any controller unit agents.

And you should be back in business.

It may also be desirable to purge the 2.2.7 upgradeInfo, but that wasn't necessary for the downgrade.