apache I believe.What is the server that you're using? apache or iis?
I was using vbulletin before so the entire structure is different.From what I think you're saying, it seems the easiest way would be to use
The Link juice keeper plugin:
http://wordpress.org/extend/plugins/link-juice-keeper/
If you mean that the file structure of the site has changed, then you would need to create/modify a .htaccess file.