With no inline admin/moderation functions there is only one way to mass move thousands topics and it is a blunt instrument. In Dashboard/Forums go to Forums and click the delete button, a dialog will open presenting you with an option to move the content from the deleted forum to a different one.

Caveats:

It works fine as long as your intention is to move ALL the content from that forum. If your intention is to mass move only some of the topics then you have to do each one manually as Alvina demonstrates in the post above. 

You will have to recreate the deleted forum if you intended to keep it sans the content. Take note of the access masks and other custom styling before deleting.

You just need to go to the topic you want to move, click on the "Tools" button and move the topic on the specific forum you like.

How to move several thousand topics from several forums to a specific forum?

Thanks.