Sorry to hear you are having difficulty with this - nice find with the article but too bad it's not a valid solution for this instance. I just spoke with an Online Engineer in Tech Support and he noted that in this instance, it looks like Tableau will likely need to intervene to change the email address for a single seat license. Please reach out to customer support and request assistance changing this setting. They will likely need to add an additional seat to your site to make the swap and then remove the old seat.
Thanks and hope this helps!