I've run the importer from classic to threads 7 more than 50 times, you shouldn't have any problem.
For any forum with less than a million posts and on a decent server you can do an import in less than 8 hours, I'd figure maybe an hour per hundred thousand posts for a educated guess.
If you really don't want to import posts, back up your non-cgi folder and prune the forums of all posts prior to today. That would be a waste, really... import
