Backups aren't guaranteed to work in future versions of ZCS.
It's probably safer (and smaller) to save exports instead. zmmailbox -z -m
foo@example.com getresturl -o /archive/foo.tgz '//?fmt=tgz'
All or portions of those can be re-imported into an empty account via postresturl or the Preferences pane of either ZWC or Zimbra Desktop.