None of the forum you linked is heavily styled or modded it seems, and actually, only the first two seems to require conversion (last two are not reachable from where I stand), since -http://www.mbarocks.com/forum/ already is running phpBB3.
Since I can't find any sub forums, I assume you are talking about the virtual folder feature of the phpBB3 mod rewrite.
So, it's as explained
here :
SeO wrote:About the old URLs, two case :
- If you where not previously using url rewriting on your board, then, there is nothing more to do than installing / activating the zero duplicate (included in the premod), and all the previous URLs will be HTTP 301 redirected to the new one.
- If you where using URL rewriting, then, post your current .htaccess and I'll tell you what rewriterule to keep in your new .htaccess in order to redirect them too with the zero duplicate.
Nothing more

There is only one extra case, you can manage to keep the exact same urls coming from our phpBB2 mods, but, converting is rather a good occasion to think about using or not the new features.
Your biggest forum has 2204 posts, so it's really not a problem to redirect all the urls.
The only difficult part is choosing the best url structure, advanced, mixed or simple, with or without virtual folder. It's worth giving it a try.
After you're done with this, the only thing to do well is to switch all the sudden with everything up and running, redirecting included, for the smoothest migration in SERPs.
We did
redirect massively on phpBB SEO if you want more clues about how it goes.
++