Do you mean the email address when you signed up? If so, yes, you can change the email address any time under Edit Profile section.
once the user creates an account, the user cannot change theh promo code any more. the promo code is only available for new users.
unless you close the account and sign up a totally new one.
Save [color=#CC0000]$97[/color] (max discount) on dreamhost plans by using promo code: [color=#CC0000]97CRAZY[/color].