You can set up a small account for your new domain and redirect visitors using:
1. HTML (meta header expiration)
2. HTML (frames also known as URL cloaking)
3. PHP (Header function)
4. Perl (Header function)
5. ASP (Header function (guess so, I don't use ASP))
6. mod_rewrite (to rewrite URLs based on a .htaccess file)
That way the URLs in your forum will remain the same.
In PHP you can do it by placing an index.php file in the new domain's account and get the following code inside your index.php:
That way whenever someone reaches the new domain they will be redirected to your regular domain.