Bob, what may have gone wrong with your first attempt at the time is that maybe you did not export the actual thumbnail (image) data. That takes up heaps of space and the exported text file could easily go over any 2GB or 4GB filesize limits. However, with offline media, you have to include those thumbnail data, or else you'll have to re-scan all those discs to re-create the thumbnails in your new database.
(which, as an aside, would give you an opportunity to check if those aging discs can actually still be read; and make backups if you're beginning to get read-errors)
Still, you should be able to export those thumbnails from TP7, maybe a few discs at a time, and import them into TP8.
Once you have done that and you want to split your new database into two (one for online media and one for offline media), you can simply exit TP and copy your database file, and then relaunch TP and use it to remove all 'online' thumbnails from the first database, and all 'offline' thumbnails from the second database.