Are you moving all of the posts and threads to the new forum (ie migrating) (it's not clear as you said the forum is nearly dead)? If you are, you can use the XenForo importer built into the software to migrate from the old to the new forum. If you aren't migrating the data you can simply start up the new forum.
As has been said, you can only run XenForo on one site unless you buy an additional licence. You will need to close down the old site and delete it after the new one is set up. You will also need to update your website information in the XenForo customer area.