Before following the steps below please do this on your test environment first as this would change all of the user logins suddenly!
Login to the management console.
Management Console>Directories>Userstore/ You will find %user_id%. Under management objects click "user". Under attributes find UID. The well known name will be
%USER_ID%
export the userstore.xml. Change the managementobjectattr for uid. Search for uid. It will look like this:
<ImsManagedObjectAttr physicalname="uid" description="User ID" displayname="User ID" valuetype="String" required="true" wellknown="%USER_ID%" maxlength="0" permission="WRITEONCE" validationruleset="User ID Validation
You will change this to use your attribute. In this case we used employeenumber.
lmsManagedObjectAttr physicalname="uid" to ImsManagedObjectAttr physicalname="employeeNumber"
Import.