Hello,
The prefix of the User Logon Name (pre-Windows 2000) cannot be changed because it is the flat name of your domain. Actually, it is not even saved to the property, only the username part is saved.
As for the domain part of the User Logon Name property, as far as we understand, you have a certain custom UPN suffix or suffixes that you use in users' logon names instead of the domain name. How many suffixes do you have? Is it only one UPN suffix that you assign to users every time or the suffixes can be different, and you need to give users a way to choose an appropriate UPN suffix?