If you need to migrate ejabberd, you need to be aware that -at least on Debian/Ubuntu- it uses your machine name as the default nodename, or to be more precise, ejabberd@hostname. This can be controlled in /etc/default/ejabberd but unless you already knew about it, it's a done deal already.
So, my naive approach of just copying files did not work. However you can use the method to migrate ejabberd data files documented in the -gasp!- manual.
You also need to be aware of the fact that your jabber domain - in my case jabber.pandemonium.de - is stored as part of the user names in your roster file, so testing it with a parallel host/domain won't do you much good. You'd have to at least simulate it with /etc/hosts entries on both client and server.