So I created a new migration that added the column and calculated the hashes for the existing email addresses in the table, however, it was not automatically creating the hashes using the current functionality.
So, it's a pretty simple call in the model, a function called before_save.
My model file ends up looking like this, which creates the hash right before it saves the email address when someone subscribes.
self.email_hash = Digest::SHA1.hexdigest(self.email + "addsometexttomakeithardtorecreate")
And that's it! An addition to the model file which is supposed to handle this sort of thing, rather than the controller.