So you should give a try to our SEO premod
, it's very handy to maintain since we provide update packages just like phpBB does. It's including all our SEO mods at once, so you can try all the mod rewrite types (not online) to choose the one you like the best.
Usually, the mixed mod with the virtual folder trick is a good thing, it depends on your topic title average quality to go for the advanced one.
About converting your old board, you'll have to take car about the required redirections if you end up using different urls with phpBB3.
It's pretty easy, you just need to keep the old rewriterules up and running at the end of your new .htaccess, the zero duplicate would do the rest (yes it's a good thing to use it;)).