While your username is in the format of an email address, it does not have to be your email address. You can change your email address at anytime from your user record. Just ensure you have access to the email address you are changing TO as a confirmation email will be sent to the the address you are changing TO and must be confirmed before change will take place.
Under Personal Setup, choose My Personal Infromation | Personal Information.
NOTE: still shows OLD email address - because new address has not been confirmed yet.
A notification of email address change is sent to the OLD email address to inform the user of the email address that the change is occuring.
A confirmation of email address change is sent to the NEW email address to ask the email addressee to verify that the change is OK. The link needs to be clicked for email address change to take effect.