Unfortunately MapQuest does not allow for an email address in an existing account to be updated. If you want to use a new email address, you'll have to register for a new account with your other email address.
Once you create a new account, you can carry over any existing My Maps if you'd like to save them in your new account.
To do this, simply open the My Map you want to save and copy the URL into a new browser where you are signed in to your new account. Once you have My Map open in your new account, you can press the "Save as My Map" button in the top left hand corner of your account.
Any saved vehicles, favorites, or home/work addresses, unfortunately cannot be carried over the same way My Maps can.