I recently switched my FAS account address to use the same address as my old disabled account (@rbarlow) used to use. In order to do this, I did need to log in to FAS as @rbarlow and change that account's e-mail to another address (because FAS enforces e-mail address uniqueness). However, after doing this, I could not log in to Pagure with this error message: "Could not set up you as a user properly, please contact an admin".

I think Pagure doesn't need to enforce uniqueness on e-mail addresses since FAS does that. That issue aside, it would have been helpful if I'd gotten a more descriptive error message.