Used to be that in the old linux days one could just use the commandline tool rename to mass-replace filenames with new ones. Amazingly, Mac OS X does not have this utility. Seems like you have to pay more and more for less and less.
After searching for a source code copy of rename (and no, I didn't want to write my own, simply because I know this script/program exists), all I found was MassReplaceIt. It works, but replace was so much easier to use.