I've found the answer to my own question, it was a little buried in this very nice [document](https://www.gluu.org/docs/cache-refresh/GluuCache-Refresh.pdf) on page 11.
What I am experiencing now is accounts created while the mapping existed only for email and not mail do not have a mail attribute even though the dual mapping is now set up and the cache refresh is still updating. New accounts and accounts that were created before the mapping changed have both mail attribute and email, etc. Is this expected behavior?