What to do with precise charms?

[Mark Shuttleworth]

The multiple-series-charm support has been added in Juju 2.0
specifically to enable upgrades of services in future. The idea is that
a charm supports a window of series, and operators can upgrade the
underlying hosts smoothly. We don't have quite all the pieces in place
yet but its close.

In general I would say move to charms that support trusty+xenial, or
just xenial.

Mark