UBB7 is a new product, from the ground up... You can't even really call it UBB.Threads (other than the fact UBB.Threads7 was the name chosen for it). When things where imported, they where assigned new id's; this counts for users, posts, forums, threads, categories, etc.
You can use the redirector scripts that come with the UBB7 import files to redirect _old_ links to _new_ links.
