RE: http://www.adobeforums.com/webx/.59b743c8/1
Download the zip file, unzip and place the script in the relevant folder.
PC:- C:/Program Files/Adobe/Adobe Photoshop CS4/Presets/Scripts/
Mac:- [hard drive]/Applications/Adobe Photoshop CS4/Presets/Scripts/
Open one of your 24bit PNG's in Photoshop, create a new action that :-
File - Scripts - savePNG8 (stop the recording)
This should have now deleted the original and saved it as the same name,same place as a PNG8 128 colours.
You can now use this action in batch.