In the Admin Center there are tools for managing both Boards and Categories.

Basically, you have two choices.

One is to rename the existing categories and boards.  When I say rename them, I am speaking of changing the Name field (what is displayed) NOT the ID fields. The ID fields are really internal to YaBB and don't affect what is displayed or how it is ordered. 

DO NOT CHANGE THE ID FIELDS!

Both Categories and Boards have a permanent ID field, and a Name field - only the Name field is ever displayed.  The ID field must be unique.

Boards can be moved to other categories, and both Categories and their Boards and be re-ordered, without any change to their contents.  Moving a Board to a different Category is done by changing the Category field in the Board Edit menu.

The second option is to create new Categories and Boards.  You can certainly go this route if you wish, and you might want to create a few new boards or consolidate some. 

You would create any new Categories first, then add any new boards.

This is done by creating the new board (it gets placed in its Category during the Board setup), and then using the Admin  Move topic tool (in the User interface) to move threads/topics to their new home.

You should probably read this section in the Codex and look at the video.

http://codex.yabbforum.com/YaBB.pl?num=1198368987/4#4

You don't say what version of YaBB, BUT - FOR SURE - BaCKUP FIRST!  (Yabb 2.4 & 2.5 have built-in backup)

Read this:
http://codex.yabbforum.com/YaBB.pl?board=backup


Good Luck

Thanks JonB, that will do just nicely!

Sorry, should have mentioned it's a YaBB 2.4 board!