upgrade path for applications series (ie. trusty to xenial)

There is a longer term project underway to accomplish this.

I am currently working on a stepping stone piece, where if you have manually updated the series of existing units, you'll be able to let juju know that new units should use this new series. You
would be able to change the series to any supported by the charm, so a 'revert' action would be
possible.

Current thinking is along the lines of:
juju upgrade-charm <application> --series <new series>

After much discussion, there is a change of plan for the juju command name to:

juju update-series <application> <series> [--force]
New units of the application will use the new series

juju update-series <machine> <series [--force]
The current machines will be marked in the db has having the new series. The charms' config changed hook will be triggered.

PR in 2.2 for juju update-series <application>...
https://github.com/juju/juju/pull/7677

Work on juju update-series <machine> has started.

PR 7677 has been merged into the develop branch for 2.3. Work will commence on the machine update piece now.

Information on how to manually update the juju database for juju 2.0 to 2.2.x will be provided.

Am I correct in assuming that the upgrade itself will be made by charms itself?

Would it make sense to add 'updgrade-series' hook, that will take care of series upgrades?

I'm asking because I can envision multiple scenarios (mainly with databases) where series upgrade can't be just done with 'do-release-upgrade' but will require coordination from the deployed units (thinking about various databases that will need to be upgraded one-by-one, esp if they are in HA setup).

juju update-series <machine> pr into develop:
https://github.com/juju/juju/pull/7776

@mariosplivalo,

For existing units, the current plan is for a qualified person to do the dist upgrade. We will be providing documentation and scripts to help migrate the juju pieces from
upstart to system for a trusty to xenial upgrade. As well as anything else which needs manual attention.

When complete, the functionality will include the config-changed hook being run as part of
'juju update-series <machine>'. The charms will evolve to look for underlying machine series in config-changed.

There is also the option, provided there are available resources, to add and remove units after running 'juju update-series <application>' to avoid doing the dist upgrade on existing units.

Doc PR on how update series for an application pre-juju2.3 : https://github.com/juju/docs/pull/2019

PR to trigger config-changed hook when a unit's series changes:
https://github.com/juju/juju/pull/7794

Doc PR on how update series for an machine pre-juju2.3 : https://github.com/juju/docs/pull/2045

All juju develop PRs have been merged.