Mike,
JFBConnect creates standard Joomla users which other extensions see and use just like normal. As long as users can create accounts on your site to begin with (and it's not required that they pay to create an account first), you shouldn't have many issues using AEC to upgrade or change a users privileges after they register.
We've had many users in the past mention that they use AEC without issue, but it's not something we test with.
Hope that helps explain, but if you have any questions or issues after you subscribe, just let us know!
Thanks,
Alex