Migrating from JFBConnect 4.1.2 J1.5.26 to J3.2 & latest JFBConnect

Can someone kindly outline what the upgrade procedure would be to migrate a JFB 4.1.2 (J1.5.26) site to J3 and the latest JFB Connect?

Moving from JFBConnect v4.1.x to JFBConnect v5.2.2 (or the upcoming v6.0) should be a simple process. You simply need to:
* Copy the database tables for JFBConnect from your Joomla 1.5 site to your new site:
#__jfbconnect_config
#__jfbconnect_user_map
#__jfbconnect_request
#__jfbconnect_notification
* After you've moved all the Joomla users over to the new site, install the new version of JFBConnect. All of your previous configuration settings and user-mappings will be imported.
That should do it. As long as your Joomla User Ids remain the same, you shouldn't have any problem. Obviously, check all of your configuration settings to make sure things behave how you'd expect.

In case we decide to use a 'staging' J3.2 server that's on a different domain, can we still setup JFB and connect it with the real domain so we can ensure everything works out before 'switching' to the new J3.2 site?

If it's a completely different domain, you should probably create a different Facebook Application for the staging domain. Everything will be have the same except that users logging in will have to approve the new FB Application. Their accounts on your site will work just the same though.

Once you go live, use the original App keys from the live site.

Also, are the user comments presented on JFB sored in the local database or are they on Facebook's servers?

The comments are stored on Facebook's servers and are tied to the URLs on your site. If your URL structure stays the same, the comments will remain. If your URLs change, the old comments will not work.

I hope that helps, but if you have any other questions, just let me know,
Alex