OK, first I'd run the following select query:
SQL
SELECT tid, title, forum_id FROM ibf_topics where title like '%DMX%'
just to make sure you had the right topics. Then, to move the topics to the new forum, you could run the following update query:
SQL
UPDATE ibf_topics SET forum_id = 'XX' where title like '%DMX%'
XX is the forum ID to which you want to move those topics.
This is untested, so no guarantees, but it *should* work.

Thank you very much.
I don't understand '%DMX%'. What does it mean ?