I was able to translate between the two database schemas enough to get the old content imported. You'll notice when you look at the comments on any of the old blog posts that the users all say "... (Guest)". That's because I never had any solid relationship between comments and users in the old system, apparently, and there was no clean way for me to determine which users were legit and which were just visitors. In the new system, the visitors actually get a new user record, which means I have a way to classify them as guests. I just had to make a broad assumption that all the comments on the old system were made by visitors, though, so they're all marked as guests now.