When you upgraded you were required to use the importer. In that zip file were replacement files for classic for redirects.
ultimatebb.cgi I believe is the name of one.
This would have been placed in the old directory for classic to replace the original file.
In that file was a couple of values to set using a text editor.
Some may need some trial and error.
In a nutshell if a user or spider has bookmarked your old urls and tries to visit the old classic url it will redirect them to the new urls used by threads.
