...OK, I think you should set up a test forum and learn the basics of mod_rewrite first before you "hack away" with mod_rewrite on your live forum.
You should just mirror your forum and then you can "hack away" and experiment with mod_rewrite and sent it links that are 404 in the main forum log file and watch the 301s in the test forum log file without doing this on your live forum (where Google is indexing you all time time).
You don't want to be experimenting on a live forum unless you are very well versed at mod_rewrite because Google, Bing, etc are indexing you in real time, and they will get more errors and bogus 301s if you are experimenting on the live forum.
What I would do if I were you is:
- Set up a test forum that mirrors your live forum.
- Look at the access.log in your live forum for 404 errors.
- Take those 404s and feed them to your test forum.
- Experiment with rewrite rules on the test forum until you are happy.
- Repeat for all 404 issues.