Well one could always mass move threads to a new forum
But then I do not know if that runs all the queries necessary to update all the tables or not.
Basically if you wanted to change a board to a new keyword you would need to do the following commands.
Lets say you wanted to change board UBB5 to Announcements
UPDATE w3t_Last SET L_Board='Announcements' WHERE L_Board='UBB5'
UPDATE w3t_Boards SET Bo_Keyword='Announcements' WHERE Bo_Keyword='UBB5'
UPDATE w3t_Moderators SET Mod_Board='Announcements' WHERE Mod_Board='UBB5'
UPDATE w3t_Posts SET B_Board='Announcements' WHERE B_Board='UBB5'
UPDATE w3t_Subscribe SET S_Board='Announcements' WHERE S_Board='UBB5'
Now I think I got them all but with anything when you modify your database backup before you try this