The existing URLs would still work.. for whichever became the 'unused' version you would just set up a simple re-direct to the equivalent URL (just adding/subtracting the www), along with a header of "permanently moved" to alert the search engines/browsers to update their links.

I have done this a few times, and it has worked seamlessly. However, there may be a bit more complexity setting it up here due to the forums software or other issues.