ubuntu@maas:~⟫ juju upgrade-charm keystone --path ~/build/charm-keystone
Added charm "local:xenial/keystone-1" to the model.
ERROR cannot upgrade application "keystone" to charm "local:xenial/keystone-1": cannot change an application's series

1 ubuntu@maas:~⟫ grep -P 'xenial|trusty|yakkety' ~/build/charm-keystone/metadata.yaml 
  - xenial
  - trusty
  - yakkety