At least with Juju 2.3.5 you can do a charm upgrade from a charmstore charm to a local charm by doing:
juju upgrade-charm --path <location> <app>
and switching from local to charmstore, or from one charmstore to another from the charmstore with
juju upgrade-charm --switch <charm> <app>
At least with Juju 2.3.5 you can do a charm upgrade from a charmstore charm to a local charm by doing:
juju upgrade-charm --path <location> <app>
and switching from local to charmstore, or from one charmstore to another from the charmstore with
juju upgrade-charm --switch <charm> <app>