Moving from Subdomain to Subfolder without loosing seo

[Snowball1]

I renamed the folder and redirected the old folder to the new one for seo purposes. Initially the url was forums.mysite.com and now the url is mysite.com/forum . I use .htaccess to redirect forums.mysite.com to mysite.com/forum. I also used ErrorDocument 404 to redirect all error pages to mysite.com/forum. My only problem now is how to redirect the old threads which are already listed in search engines to my new urls. I have several pages indexed in search engines and I don't want to loose seo. is there a way to redirect all the old threads to their new threads without loosing seo? Thanks