I came across this program when i needed to rename hundreds of textures for a race -> race Conversion and decided to share it.
Firstly download the program from here http://www.albert.nu/programs/renamer/renamer60.exe
Installation pretty straight forward.
The Renamer looks like this
Next you have to open the directory of the textures that you want to rename. In my case it was "Character/Human/Male" you should see a list of all your textures. Now bring up the "Replace" Menu (CTRL+R) or manually select it. Next type the current name of the files into the "Replace" box. So in my case this was "Human". Also write the name of the race that you want to change the textures to. In my case "Troll". Click ok.
The files should now look like this. However they havent changed yet. Click the button to change all the files. Walah you have successfully changed all 263 Items in the list to "Troll******"
This guide does not tell you how to change races as i am still trying to fine tune tuning the files into an MPQ but this is by far the easiest way of renaming the files within directories
Cheers,
Dave