Just an FYI for something that bit me in the Admin UI when editing a user alias (the users COS reverted to the 'default' COS without me touching the COS field).
This is quite odd, but if you have a user assigned to a COS other than 'default', and you have more than one domain, if you go to edit the user, then go to the 'aliases' tab, if you click the drop-down for the domain and select a domain other than the domain for the user you are editing (so the 'remove' and 'add alias' buttons appear), then flip back to the 'General Information' tab, you will see that the user's COS has changed back to 'default'.
If you aren't looking for this, you will inadvertently set a user back to default COS. In my case, I had users in a COS called 'mobile' and I only discovered this when they couldn't use their smartphone to connect to zimbra mobile because the COS changed.
Here is the bug I filed, but I wanted to post here to make folks aware of it: