Convert entire iTunes library to MP3 then delete old version? Is there an easy way to Convert an entire iTunes library to MP3 format then delete old version? I know you can do it manually, but then you have to go check every other song and delete it, is there a fast and easy way?
I would select all the music you want converted, and make a playlist out of them. Next convert the files. Then select all the songs in your new playlist and delete them from your library by pressing Option Delete (OS X) or Shift Delete (Windows).

Additionally, I hope you aren't converting lossy music to MP3, as that would produce lesser quality files for no benefit.