1) Specify the new subdomain path as being one level above your threads folder, and then you'll be able to connect to it via...
http://forums.domain.com/forums/ and
http://www.domain.com/forums/ ...at the same time.
2) You would also need to change all instances of...
http://www.domain.com to
http://forums.domain.com ...in cong.inc.php and pals/configpal.php (if IIP is installed)
This would mean that the corporate users would obviously have to use the subdomain URL from now on to connect though, and not the one being used presently.
The regular users however shouldn't be affected by any of this.