EDIT: The script to do this automatically is complete and in place. Click and forget.
In links pointing to old board messages, there is a "num=" in the URL and sometimes a "start=".
The new board uses the "topic=X.Y" where X is the topic number and Y is the same as the "start=" number.
The topic numbers aren't the same, but I have a cross reference table that tells the new topic number based on the old topic number. The file is attached.
For example, using the attached cross reference list I can convert an old URL as shown:
because 1109989865 cross-references to 2269.
I hope to automate this conversion, but if you're hard up to find where an old link is you can look it up using the attached cross-reference file. (Automated script is in place now.)