Internally in OnTime Group Calendar users are identified by a so-called OnTime ID which is a short, unique, ID. The OnTime ID first created for a user when he/she is brought into OnTime Group Calendar and is thereafter maintained by the administration task.
For each user OnTime records the fully distinguished name (DN, e.g. CN=John Doe/OU=Sales/O=Acme) as well as the UnID of the person document. Whenever a user is processed as part of the administration task the user in OnTime can be found using either the DN or the UnID. This means that a user in OnTime will keep their OnTime ID even though that are re-certified (get new name, new organisational hierarchy) or a new person document is created for the same user (same DN). Since the OnTime ID stays the same the user also keeps their private groups.
It follows from the above, that a user cannot change both DN and Domino Directory UnID at the same time while keeping their OnTime ID.