Change Primary E-mail Address of Existing User - Hosted Exchange

[James]

Command:
Set-Mailbox <Identity> -EmailAddress SMTP:<New primary e-mail address>,<User ID>,<Existing Proxy Address 1>,<Existing Proxy Address 2>,<Existing Proxy Address n...>


So, if you currently have configured all the users in your domain (ie: MYDOMAIN.COM) and want to change the address of a currently registered user (ie: From John@MYDOMAIN.COM to JohnDoe@MYDOMAIN.COM) run the following command:

Set-Mailbox John@MYDOMAIN.COM -EmailAddress SMTP:JohnDoe@MYDOMAIN.COM, John@MYDOMAIN.COM

This will change the primary email, however it will also allow emails to be received from the old address as well.