\label{ForceIDChange}
Although the username is the principal unique identifier for a user in LON-CAPA, the student/employee ID
is a secondary identifier, which needs to be defined (and unique) for certain applications.

The default is not to allow the student/employee ID for an existing user to be changed when
modifying a user's information in course, author or domain context.

This default can be overridden by checking the ``Force change of existing ID'' checkbox. If that is
done the old ID will be replaced with the new ID. (Note: the new ID can not be blank; i.e., once an ID
has been set for a user, it can be changed to a different ID, but not set to be null).

A side effect of updating a student/employee ID is that the database file used for the quick
look-up of username for a specified student/employee ID will be updated. This can potentially impact 
bubblesheet grading, if there is a possibility that student/employee IDs are not unique.

However, there is no requirement to set a student/employee ID for a user, and you will not encounter
issues doing that unless you attempt bubblesheet grading or uploading scores from a csv file 
containing IDs and points as column data. If either of those actions are likely to occur in courses
in your domain you should attempt to ensure that student/employee IDs are unique.

Your domain may have been configured to automatically update student/employee IDs from an institutional
data source. Similarly, IDs may be retrieved (and set) automatically when creating a new user, if the
username is for an institutional user, and your domain has been configured to support that.

Individual courses contain their own database files of class rosters, which include a copy of each
student's ID. If a student/employee ID is changed by a Domain Coordinator, there is an option to
propagate that change to the individual classlist database files for courses in which the student has
a role that is currently active, or one that will become active in the future.