JFBConnect has an option to import any previous connections made in JomSocial. With that, your users shouldn't even notice that they're logging in with a different extensions (unless you're asking for more permissions for additional profile information that JFBConnect supports which JomSocial does not).
Beyond that, we have multiple options for existing users to connect their accounts to any of the social networks we support. The linking can happen if their social network email address is the same as an existing Joomla user on your site, or they can login with their existing account and use the SCLogin module to authenticate and connect to another social network, or various other ways.
I hope that helps, but if you need anything else, just let us know.
Finally, right now, we're having a $10 off discount of all new subscriptions. The last promotion we ran was in November of 2013 (a year and a half ago), so if you're interested, nows the time to try JFBConnect out.
Thanks,
Alex