For people moving half a million posts from another version of forum software (let's say vB3 for example) the ideal would be to keep the URLs as they were. No way Google will re-index deeply buried stuff on most forums if the URLs are changed, and 301 redirects would be very server intensive on an existing forum.
Pre-written htaccess stuff to allow people to keep their existing URLs whatever software they migrated from might be a good selling point to anyone who has met an SEO. Easy enough to do if the thread number is in the URL (which it seems to be).
Is the dot in the 'friendly' URLs not a reserved character (used to denote file extensions)? Dangerous stuff - really ought to be using non-reserved characters there.