Batch conversions of MP3 bit rates

I'm trying to convert large batches of MP3 files to a lower bit rate while preserving the file name and path. AudioCommander 3.1 seemed promising, with it's "Use Source File's Path for Output", "Overwrite Existing Files" and "Delete Source File After Conversion" options. Unfortunately, whether used alone, or inconjunction with either of the other options yeilded undesireable results. The first two options together would have been ideal, but I think because I was trying to convert from MP3 to MP3 (just changing the bit rate), vs. WMA to MP3 or something, I got an error stating file could not be opened. When using the first option with the third option, it created converted copies with appended file names, and DID NOT delete the original source file. With thousands of files to convert, having to manually delete source files, and rename the newly created converted files would take me forever. Any software suggestions?