How can I change the eMail ID of my Rediffmail eMail account?
from the Rediff Email FAQ:
Can I change my administrative email ID?
Yes, you can change the username of the administrative email address at anytime. All you need to do is create your desired ID and send us a mail from your administrative ID asking for the change. After we send you confirmation of the same, you may delete the unwanted ID if you so desire.
Can I change the other email ID's I have created?
Yes, you can change/delete the username's of any of your other email ID's.
Login using your administrative email ID. Click on 'preferences', go to 'add/delete addresses' to change/delete email ID you don't require any more. You can replace the deleted IDs with a new one, if you desire.