My experience is that when there's a major change in a large forum community, several derivitive sites pop up fairly quickly. The overall traffic within a community goes up, but it's spread across lots of sites. Eventually the derivatives die out slowly because they fail to achieve a critical mass or don't have the right atmosphere and focus.

I've been involved with a Van Halen site for four years. It's the largest, best run VH forum. We've seen splinter groups form their own sites because of personal agendas or disagreements. This has happened dozens of times. The band itself created a forum, and that went the way of the rest as well. Each time our posting numbers dropped to a degree, and then returned after a short period.

The one really cool thing about these kinds of changes is that it allows people who might not have felt comfortable posting in the past to become more involved. It's always brought out great new contributers on that VH site, and I can already see that it's bringing out some new faces here, too.