From Online Manual
To move a topic, go to the topic that you want to move and click Move Topic. You can then choose which board you would like to move the topic to.
If you do not want to post a redirection topic, uncheck the box that says Post a redirection topic before you select Move Topic.
A redirection topic automatically includes the original name of the topic, along with a link to the board it was moved to and a link to the newly moved topic. You do not need to type this information into the fields [BOARD] and [TOPIC LINK]. By default, redirection topics are locked.
If you want to change the name of the topic that is being moved, check the box next to Change the topic's subject. You will then see a field where you will be able to enter a new subject. You will also have the option to make all the message subjects the same as the one you chose for the new subject by checking the box next to Change every message's subject.
Once you have made the necessary changes, click Move Topic.
Mass Moving Topics
If you to move a large number of topics out of a board, you can mass move topics from one board to another board by going to the message index of a board and using quick-moderation. Select the checkboxes to the right of every topic that you wish to move and then go down to the bottom of the page where you see a dropdown menu. Select Move selected to and in the dropdown menu to the right select the board to which you wish to move the topics.
If you cannot see any checkboxes, you will need to enable them by going to Profile > Look and Layout. To the right of Show quick-moderation on Message index as, you will see a drop-down box. Click on that box, select the option that says checkboxes, and click Change profile.