If you use the PATCH command (modify) of the Kelvin API to change a username, the counters that take care for the uniqueness of the usernames are not triggerd!
If the same username will be calculated in a following import (create) this creation/import will fail.
The counter logic is only implemented in UCS@school environments.
As you surly will keep every userdata it could be a good idea to use a high number in the username, where the counter would be placed manually. Alternativ you could create the user and copy the data manually. A new password will be set.
It might be possible to trigger the counterlogic during a patch command via hooks. If you want that and need help by doing that ask your sales contact.