You do not mention what version of exchange you are using. Please include details when you ask questions. We can answer with more detail when we know what you are using.
When a user changes their last name you do not need to create a new mail account. The best thing to do is change the last name on their existing account, then add a new SMTP address and make it primary. You should keep the old SMTP email addresses so that they can still receive mail from people who do not yet know the name has changed. The details of how to do this depends again on what version of Exchange you are using.
I am using exchange 2003 on a Windows 2003 server