Primary email addresses cannot be replaced unless you cancel the subscription associated with it then sign up with a new email address.
If you would like to pursue with changing the primary email address, please contact MSN Cancellations, through this link: https://support.msn.com/default.aspx?page=msn_support_task_cancelaccount_msnclient
You may also refer to the Private Message below for the contact details.
Another option is to add a secondary email address. You can follow the steps on this link to do so: http://answers.msn.com/solution.aspx?solutionid=3dc456fb-adf6-4492-86ab-a30285ea9a5a
Feel free to reply to this post for any questions and feedback.
Thanks!