I'm in the process of upgrading my forum with a new style and a few mods to include a SEO mod. The forum is heavily indexed by google, it's several years old (about 5) and a PR 4. If I add a SEO mod that changes the names of the links from "/viewtopic.php?t=293842&f=23" to something like "/excellent_thread" would that hurt my SEO if the pages are already indexed? Also, will changing up my site design hurt SEO at all?
There will probably be a temporary drop while the new pages are being indexed. The "/excellent_thread" type URLs are much better and will bet more pages better rankings - buy you must 301 the old pages over to the new URLs to that you won't lose any incoming link juice
Site design won't hurt if you keep contents like: title, urls, meta tags etc. intact and just change th layout of the site.
There is the potential for collateral damage, but if you're careful (such as using 301 redirects and providing meaningful semantic markkup with the keywords in the right places), then you should be fine. Any drop you see would then be temporary.