It's way too early to tell what if any impact the change in software will have on site usage. But lets look at some numbers anyway.
First off, early indications are that everyone has figured out how to post - new posts figures for the last couple of days are broadly in line with the same days last week. Subtract all the posts made in the topic about the move (which don't really count as normal posting) and you've got figures down a bit, but not to a worrying extent.
There are page redirects in place to bring anyone attempting to visit old forum content pages to the right place on the new system, so search engine traffic is still finding us. It has dropped by about 10% on the same day last week, presumably as Google and the rest update the index with the new urls. That kind of drop is well within the realms of random Internet fluctuations anyway.
And it's maybe a bit early, but it looks like Googlebot is finding the new pages easier to eat - this shows how long on average it takes to download a page. There's not a lot of value in that information, but it indicates that the new scripts are at least not running any slower than the old ones.
And if anyone wants to do their bit to boost the stats - get posting. Posts generate more posts, and more visitors.