Hmm, that's odd. Gaps in the forums shouldn't matter, I've done a few imports with some pretty big gaps. The forum numbers, categories numbers, etc. will definitely change when an import is done. They won't be the same as what they were in .classic. This is totally normal behaviour. You definitely shouldn't have to, nor want to, alter those at all since everything is tied to the id in the database. Changing any of those is definitely going to cause an issue.
The only thing I've had to fix after an import is where no forums are showing up. This is a small bug on some setups in the importer that isn't assigning the forums to a category and usually fixable with a quick query in the database if I can get the admin username/password. The problem is since you've already changed all of the forum #s I'm not entirely sure your current database is fixable.
