Please make sure the plugin "auto-subscribe during Joomla registration" is enabled. This plugin will take care of the synchronisation.
Make sure that the new e-mail address does not exists as an other AcyMailing subscriber otherwise Acy may not be able to update it on its main profile
Once done, please go on the Acy import screen and import your Joomla Users to trigger a synchronisation. You should now see the right e-mail address on his profile...
Also, the user may have to log out and then log in again to reset his user session otherwise the session values will still contain the old values.