The old modules should still work, as you noticed. However, it's definitely best to start migrating them to the SCSocialWidgets. There are some updates in the newer releases for bug-fixes and improvements that only the SCSocialWidget implemented modules would have in them.
I'd recommend simply installing JFBConnect v6, which will add the new SCSocialWidget module and upgrade everything that's necessary. Then, as you move your modules to the SCSocialWidget, delete the old modules from your module manager and go to the Joomla Installer -> Manage area and uninstall the older JFBConnect v5 type modules from there.. that way, there's no old code hanging around and you won't see those modules as an option when you try to create a new one.
I hope that helps explain. If you have any questions or issues though, just let me know.
Thanks,
Alex
Modules: coming from v5.x.x
