In that version there is no admin tool for mass moving, so you'd need to do a couple direct SQL commands. I'd recommend doing a database backup before proceeding.
First, you'll need the forum id of the forum you're deleting and then the forum id that you want to move the posts to.
update w3t_Posts set B_Board='newid' where B_Board='oldid'
You'll also want to update the total number of posts and replies in the new forum. So you'll want to the total that exist in the forum you're deleting. In the query below, #posts is the total # of posts in the board you're deleting, #threads is the total # of topics/threads, newid is the forum that you've moved them to.
update w3t_Boards set Bo_Total = Bo_Total + '#posts', Bo_Threads = Bo_Threads + '#threads' where Bo_Keyword = 'newid'