The old links will still work if you use the redirector scripts that come with the importer. As this is a new product, all of the old links will cease to function (with the exception of if you choose to use the redirector files to keep old links alive).
The redirectors are the files ultimatebb.cgi and ultimatebb.php that are in the file with the importer file for classic to threads; it's a dead simple install, simply update the forumurl in the scripts, then overright your old files on yoru old forum install and the old links will forward to the new locations.
