The examples for export/import on http://continuum.apache.org/docs/1.2.3/installation/upgrade.html only use one parameter of -buildJdbcUrl or -usersJdbcUrl.
Thus users might use two separate runs in order to backup the databases. However, then the export fails (as the user export requires the key.xml).
When both options are specified, no problems occur.
Furthermore the paths for the databases changed in continuum 12.3.
Instead of data/continuum/database and data/users/database the paths are now data/databases/continuum and data/databases/users, respectively.