Unfortunately there isn't a really good option for this. We currently don't have a command line version available for this.
One option would be to make a copy of your current classic forum files (the non-cgi files). You'd need to make an anouncement that any new users or posts made after that time until the import is finished will not be carried over.
Then you could run the importer and when it asks for your non-ubb path, you can put in the path to the copy that you to be assured there is no corruption. (One of the possible problems when running the importer on a live forum). It's ok to use the live copy of everything else, just the forum ones are updated quite often so possible problems with those.
Not the best option obviously since A) You'd need to make a copy of the files, so that's quite a bit of space needed and B) you'd have a couple of days where things wouldn't be carried over to the new forum.
