Comment 1 for bug 1704135

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>