Your Gravatar account's email address needs to be the same as the email you use on your DB account. So all you need to do is go to your account settings, and change the email there to the one that your new Gravatar account has. After you do that, any avatar you set with your new Gravatar account will appear on your profile.
Most sites have a "forgot password" button that you can click to restore your password. It'll send an email with your password to the email account you made the account with, so if you want to keep your first Gravatar account, you can do that.
