The same problem has been discussed rudimentaly in this thread
http://www.simplemachines.org/community/index.php?topic=29522.15
Fact is: I upgrade from yabbse in spring 2005. The search engine position of my "new forum" (same content) went down and down. I guess (or I'm quite sure) the reason could be duplicate content.
.../index.php?topic=532.msg1735
.../index.php?topic=532
etc.
Lainaa
from http://www.simplemachines.org/community/index.php?topic=29522.15
It's possible to make them look the same, I suppose, by editing SSI.php... but the whole point is that they do link to different things.
The different urls may link to different things but to the same code. And that's the main aspect for search engines. Maybe anybody could find / had found a solution ... looking of urls in relation of the user-agent?
Google hates it when your links change. Updating to SMF made all of your links change. In a little while, it will recover.
-[Unknown]
I agree - it's because all the links have changed and Google et al have to update their index to take account of all the changed URLs.
In such instances, it's always worth ensuring that you have a decent 404 for human users to find what they are looking for (ie, a mini-sitemap) or else simply redirect 404 traffic to your homepage.