If you had a lot of threads in the old forum and your new forum cannot publish the exact same URLs there is no practical method to accomplish what you're asking.
I would do a RedirectMatch on the old forum URL and point it to a new thread that explains the change.
The search engines will crawl your site and reindex everything.
What is important is that you not simply abandon all those old URLs; you need the internal PageRank (not the value you see in the toolbar) to drive crawl to your new forums.
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