Any movement on this?
Loss of Google juice might be starting to occur. Depending on how your users are searching -- in this case, if you look at your Google Analytics, your incoming search traffic might be already much lower to "urbantoronto.ca/forum" (or with an unusually high bounce rate, due to "Not Found") and will be lowered for about 1-2 years if you've already seeing fewer visits coming from Google, since "veteran-ness" of links is extremely important. It's always a good idea to write a URL rewriter that preserves the old forum thread URLs to automatically redirect to the new forum URLs. You may be running out of time to do this.
This is your most important ad-revenue-resurrecting SEO move; if you notice your searches towards UrbanToronto Forums going down at all, this is possibly 90% of the cause.
Google still memorizes the old links for the moment, e.g. in an exact-string search
example. Click either UrbanToronto search result, and you get "
Not Found". If you write a URL rewriter for old forum URL format to new forum URL format (e.g. .htaccess file or php script etc) you could perhaps resurrect at least some of that google juice since Google keeps broken links for some time. This will more quickly re-rank your page rank upwards somewhat, even for unrelated searches, because those are veteran URLs, some that are many years old and now suddenly 404.
Citation: I run a blog site that earns advertising revenue, and learned the hard way. And "Google Panda" maulings are no fun.