And welcome
It is pretty easy to redirect all the older urls using the
zero duplicate together with one of our mod rewrites (currently, only the
advanced one was released, but the other are scheduled).
It will simply redirect all possible duplicates, including the older rewritten urls. All you'll have to do is to keep the rewriterules from the previous mod rewrite to allow the zero dupe to take care of them.
The method was already widely tested, it's SEO safe, you should keep backlinks and Pagerank, with most likely no loss of visitors during the massive redirecting.
You should test a bit before you go online with it, just to make sure everything is working as expected, especially the headers returned by your server. Everything is explained in the install files.
It's simple in the end, but it's important that both the new urls and the redirecting works to optimise the migration.
As well, we'll be soon releasing several versions of the phpBB SEO mod rewrites for phpBB3, including a simple one that could keep the exact same url as you do now very easily.
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