The email account is only used for verification during the account creation phase, so after the client’s logon account has been set up, the email address is simply the account username. If your owner wants to use their new email address as the account username, they simply create a new account and link their profile to the new account. They can then delete their old account if they wish.
The only consideration with keeping an old email address as the username is if the owner forgets their password and needs a password reminder. If the owner no longer has access to the email account, they will not receive the password reminder.
A logon account can link to an unlimited number of statement accounts from an unlimited number of property managers, and an unlimited number of logon accounts can link to any one statement account.