Well, you can easily redirect them all with the zero duplicate mod.

Converting would be a great occasion to do it since you'll have to redirect in all cases.

With the zero duplicate mod, all you need to do to properly redirect everything is to keep the old rewriterules up and running at the bottom of your new .htaccess and voilà

++

Ok, I will try

So, I need to install this mods, right?
http://www.phpbb-seo.com/boards/phpbb-s ... -vt66.html (Simple, Mixed or Advanced?)
http://www.phpbb-seo.com/boards/zero-duplicate-vf45/ (Simple, Mixed or Advanced?)

Well, you could take this occasion to first work on converting your forum, at least, if you plan to do it within the next couple month, you should wait until you do so, it would save some redirecting and would be better for both SEO and your forum.

Give a try to our SEO premod, it's installed as fast as phpBB and you'll be able to try all our phpbb3 mods at once.

Otherwise, you need the mixed version of the zero duplicate with the mixed mod rewrite and so on