How to find old forum content with old links (URLs)

Because forums software changed, some external links are not functional.
It is possible to manually navigate to old content by modifying the old link:


After (current forum): 

There seems to be one problem, though - it in above example "discussion" is the name of a section and I'm not sure how that can be derived from the old link.

If you have access to the server logs, you might find the top 5 or 10 broken links and be able to do http redirects for those. Otherwise, the search feature and maybe google doing its natural updating should get the rest of people where they want to go.