The permalinks for the URLs were changed, breaking every reference.
Every forum now has its associated number tied to it - e.g.,
https://www.bikeforums.net/classic-vintage/... has now become
https://www.bikeforums.net/classic-vintage-181/...
This is going to wreak havoc with existing links w/o some sort of redirects to fix it, plus it's going to hurt Google SEO beyond belief.
-Kurt