do you want to change the xmpp.domain and use a new one? You could still use the old name as an alias name.
So you can name your server “foobar” and use as xmpp.domain “example.com” without problems.
if you really want to change the xmpp.domain you probably need to inform your users that you are using a new one. I did never do this, replacing all occurrences of old-xmpp-domain with new-xmpp-domain in the database will do the job. The user export/import plugin should be an alternative, then you need to change the xml file before importing it into the new server - all logged chats are lost using this way.