Go to http://setiathome.berkeley.edu/get_passwd.php, fill in the email address you want to change to in the Email address option, then click OK.
Log in to that email address, check for the email from Seti. It may be in your spam folder, check there as well. Click the link in that email, you'll arrive here at whichever other account you may have, which is registered to that email address.
Change its email address to something else, it may be bogus, a non-existing address. Save changes. Log out of that account, then go to http://setiathome.berkeley.edu/home.php to log back in, but this time add the information for the account you posted this thread with.
Now you can change the email address in that account to the one you just freed. No two accounts can use the same email address, it is the unique identifier. 27 million people can call themselves Batai, but only of of those will have the email address your account got registered with: you.
Fighting for the correct use of the apostrophe, together with Weird Al Yankovic