Comment 4 for bug 1530831

Apologies for the private link. Here's a public paste of the relevant part of that code - "update_deployed_services".

http://pastebin.ubuntu.com/14687410/

It does the following:

- Back-up existing local charms
- Go through each service and
  > Upgrade its charm if it changed
  > Update its settings if they've changed

It does the charm-upgrade and the settings-update (mentioned here: https://bugs.launchpad.net/mojo/+bug/1524872) together, because in some situations you may have new settings which only work with the new charm version and would break the old one, or the new charm might not work without some additional settings - so updating the two as close to each other as possible seems safest.