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Personally, and this is nothing to do with arguments for and against, I would never use any extension that changes links due to the fact that there is always a possibility that at some point in the future those links stop working and it could break your board.
David Remember: You only know what you know and - you don't know what you don't know! My CDB Contributions | How to install an extension I will not be accepting translations for any of my extensions in Github - please post any translations in the appropriate topic. No support requests via PM or email as they will be ignored
we understand exactly what you are saying and what you are trying to do. the problem is that you are wrong in your belief about the search engines and how they see dynamic links like phpbb uses. there is no problem of duplicate content at all for the search engines. they understand how it works period.
I'm baaaaaccckkkk. still doing work on donation basis. PM your needs.
If you really want to re write links, there are the 2 extensions Toxyy linked above, but be aware that (as others have pointed out) there may be future problem if the extension is no longer developed and/or supported.
Also any search engine ranking you already have will probably be lost as the links will all change.
As Robert said above Google et al understand dynamic links and know how to cope with so called duplicate content.
You say you are out of this topic, so I am locking