After migration of users the logon name for a user cannot be changed and the following error is displayed:
The specified user logon name already exists in the enterprise. Specify a new one, either by changing the prefix or selecting a different suffix from the list.
This is because the logon name trying to be specified has already been used by another account in the enterprise.
Choose a different logon name or change the other logon name so you can use this logon name for this account.