Did the post IDs actually change, or just the links? It's real easy if the post numbers are the same.
No, the post ids changed too.
In order for a correlation to happen, one would have to have access to the old forum database and the new forum database. You could suck the post IDs, usernames, timestamps, and maybe the source BBcode from the old forum database, then match it up to posts in the new forums to have a post number to post number map.
Then you could write a script on the web server that, when it sees a link to the old forum URL (i.e. /ubbthreads/
stuff), it looks up the new post number and issues an HTTP 301 to the new correct URL.
I could do it if they let me have access to the old forum database and new forum database and their forum web server, but I
really doubt that I have that kind of influence.