Replace semicolon with a comma in the COMPOSER field


I have more than 20,000 MP3 files and WMP11. During playback, for example, WMP11 displays John Lennon [Composer] when in fact the composer is actually John Lennon; Paul McCartney. I would like to convert the semicolons to commas on all 20,000 files. All of these files are stored in subfolders on a single drive, and I wanted to know if there was a DOS batch command to perform this action. Also, would there be any disadvantages (as far as WMP11 is concerned) to getting rid of the semicolons?