How do I change User Mail Account?

How do I change User Mail Account?

I am a domain administrator.  When I set up the first users, I set the email as [firstname].[lastname]@[mydomain].com.  Now we have decided to simply use first names instead of firstname.lastname as the email address.

When I log in to the Control Panel and select User Details / Mail Accounts, I see the original firstname.lastname email address.  I have already changed the email login id's on User Details, but it still shows the original email address in Mail Accounts.

When I attempt to compose a message in my user account, it shows that it wants to send the message as the original account.  How do I change the mail account to show the new way we want it to be?