Comment 6 for bug 1743901

Note the official mongo upgrade steps are:
https://docs.mongodb.com/manual/release-notes/2.6-upgrade/
https://docs.mongodb.com/manual/release-notes/3.0-upgrade/
https://docs.mongodb.com/manual/release-notes/3.2-upgrade/

All of those say "to upgrade to 2.6 you must be running 2.4", "to upgrade to 3.0 you must be running 2.6", "to upgrade to 3.2 you must be running 3.0".
And there is one extra wrinkle that 2.6 introduced a different user authentication mechanism than 2.4. And 3.0 only supports the new one. So you have to go from 2.4 to 2.6, then fix the user authentication, and then go 2.6 to 3.0, and then 3.0 to 3.2.

The actual *Juju* recommendation for how to handle this is to bootstrap a new controller and then migrate the models to the Xenial one.