According to google, its better to use dynamic urls for dynamic software like smf.
The ONLY possible benefit is when users give you a backlink without a title, where the keywords in the url MIGHT help.
<a href="
http://www.yoursite.com">
http://www.yoursite.com/</a>
So I suppose it comes down to whether you find pretty urls pretty or unpretty. Chose which you prefer.
As for changing them, there will be a negative seo impact in changing to ANY form of urls.
This is because it takes search engines a while to dump all of the old style urls out of its index, and replacing with the new type.
It is important where possible to redirect old versions of urls to new versions (best using .htaccess)