So you are trying to change firstname.lastname@example.org to email@example.com then?
I think it will have to be a case of removing the user and then re subscribing them back to the list then in that case.
So you would put firstname.lastname@example.org [name: fred] in the remove box and remove that user.
Then put email@example.com [name: john] in the add box and add them in. A subscription message will still be sent out and they will have to click the link. It has to do with the DreamHost strict anti-spam policy of course.