I guess I just don't get it. How would all my currently indexed links still work if the new URL's to the same topics are different?

As I see it, unless there is still a file named ultimatebb.cgi with a script that translates the incoming URL and redirects the user to the new page, users will get a 404...right?

I understand what you mean about the new URL's getting indexed by engines(eventually), but I want to keep all my existing indexed URL's active indefinitely.

Any thoughts about writing a version of ultimatebb.cgi that can be put into sites that convert to threads specifically for this purpose?