Hi,
Unfortunately(as far I knew) your C user ID can't be migrated right now. I also had experienced the same problem earlier.
You can not even change the email ID as it will be in display mode ( when you try to edit the email ID in your reputation)
But you can still logon to SCN with your sap ID. In case if you are logging in with single sign-on now, cancel the single signon or open in Firefox browser and try logging with your sap ID.
But as you will be leaving the company you may not get notification mails. Try creating rules in Outlook to forward mails to your personal mail ID( Not sure whether this will work)
Regards,
Kiran