Comment 2 for bug 1905056

It actually allows downgrades unconditionally which is different from my expectation.

$ juju deploy ./bundle_ubuntu.yaml

$ juju upgrade-charm ubuntu
Added charm "cs:ubuntu-16" to the model.

$ juju deploy ./bundle_ubuntu.yaml
Resolving charm: cs:ubuntu-15
Executing changes:
- upload charm cs:ubuntu-15 for series bionic
- upgrade ubuntu to use charm cs:ubuntu-15 for series bionic
Deploy of bundle completed.

^^^ doesn't warn or stop even though cs:ubuntu-15 is a downgrade.