[Logician]

an SQL command can help:
[sql]
UPDATE thread SET forumid=X WHERE forumid!=X AND open=0
[/sql]
(Replace X with your forumid where your threads will be moved.)

Please Notice:
1- I havent test it out and considering the powerful effects of such a query, please apply it to your test board first or backup your board before using it! I dont accept responsibility although I believe it will work without any problems.
2- This will move ALL your closed threads into that forum, if you want to exlude some in a forum Y add " forumid!=Y AND " after "forumid!=X AND".

Considering the fact that it works, you can trigger it inside your vb files like:

PHP Code:

$DB_site->query("UPDATE thread SET forumid=X WHERE forumid!=X AND open=0
"); 


If you add this line to your vbulletin files, it will automatically run when that file is run. For example you might consider putting it in register.php and whenever a new user registers, your threads will be moved too..